As for Google, the only connection to the SW is some limited use of RDFa [1], which as far as I've heard doesn't impact search ranking. As well it shouldn't --- that would be easy for anyone to game to get a higher ranking, just by adding some gibberish semantic tagging.

[1] http://rdfa.info/2009/05/12/google-announces-support-for-rdfa/

I will only comment on Google, since Bing is a big unknown in everything: Currently, microformats are partially supported by Google.

hReview, hCard etc. are supported: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=146897

You can check how will the webpage with microformats included show in Google: http://www.google.com/webmasters/tools/richsnippets (they don't guarantee it but they have already been showing rich snippets for LinkedIn and Facebook)

As for your question - Google is continuously adding new rankings factors to their algorithm. Since microformats (and possible RDF, too) are new and underused (i.e. not known by spammers, yet), it is possible that Google will give heavier weight to the pages containing microformats or RDF (when they know how to handle it).

BTW: Rich snippets will certainly increase click-through rate for the search engine results that contain it.

Everything else is speculation.

Tagging of content for search engine ranking should just be about following the best practice of POSH and writing relevant useful content. The semantic web isn't really about search at all so your question isn't easily answerable. Search is about looking for a document or page that contains the answer you are looking for. The semantic web is more about providing the actual answer to your question or providing assistance in making decisions about something.

To understand the difference suppose you were trying to find the date a particular book was published. You could use a search engine to find a web page that you would read to learn the answer. The semantic web aims to allow you to ask your question and get the answer directly without having to scan through a web page.

Update: Google state that rich snippets do not influence search rankings. See http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=99170

Just for the sake of completeness as it hasn't been mentioned yet, Yahoo! recognizes and extracts RDFa, but they say explicitly that it doesn't influence the ranking in any way. See their FAQ about SearchMonkey and the Semantic Web for more information.

There's a difference between semantic tagging (metadata categories for the page) and embedding structured data. Simple tagging is very unlikely to affect ranking any more than existing meta tags do - though it can give additional search terms and support the filtering/facet browse interfaces offered by some of the emerging search engines.

However, for embedded structured data there is actual evidence, not just speculation, that this affects ranking already. Specifically Best Buy reported at the Search Engines Strategy conference late last year that the pages which they had marked up with GoodRelations RDFa data jumped significantly up the Google results order and showed a 30% jump in traffic - Scott Brinker blog item. So this is not a controlled experiment and ranking algorithms change all the time so if it is was true late last year it may not remain true. Never the less, an interesting datapoint.

Let's hope semantic search engines won't be so naive that it will be worth abusing semantic tags to get higher search rankings. I'd hate semantic tags to go the way html meta keywords tags did.

A semantic tag in a page adds more information to the page, but hidden from a human. It should allow a person to read an article normally but give more data points to machines which would otherwise struggle with natural language processing.

Simply, more data should mean better results. It might help a search engine work out that there are a number of independent groups of similar values for a common search term (e.g. "jaguar" - car or animal?), and help provide disambiguation to searchers. This should mean less bandwidth wasted by your car dealership on folks looking for big cats.

The next stage is for search engines to present a direct answer to a question (e.g. Wolfram Alpha) and take them to only the webpages providing this exact answer. To do this the search engine needs to have some confidence in the data within a page. Search engines already give a value of confidence on a webpage level (Google PageRank for example) and to get direct answers they now have to drill down into a web page and give confidence to individual words and numbers within it.

For the car dealership example, tagging your showroom locations and products might allow them to be found on Google Products and all its competitors - without ever having to explicitly upload or register your publicly available product data to any site but your own.

I'm not aware of any direct effects of semantic tagging on the ranking methods of the big search engines. But I think there could be an indirect beneficial effect of adding semantic markup to your pages. If the RDF in your page is discovered and used by linked data consuming apps, then it is likely to lead to more people finding your website and linking to it, so increasing your search rank in the 'normal' way.

This is just an extension of the "produce useful stuff" school of SEO.

I think people worry too much about postulated factors inside the Google search engine, rather than about being part of the web community.

An emerging ecosystem of content syndication systems are going to emerge based on the semantic web; isn't that the whole point of it? Semantically tagged content is going to get picked up by those systems, and that's going to lead to two things: (i) traffic, and (ii) links.

The first is measurable (unlike postulated ranking factors) and the second is certainly known to improve your rankings in the long term.

Another area of semantic technology is using semantic information to "get more" out of unstructured text and links. I'm already finding this quite useful in competition research and other areas of SEO.