There is a new download at Forum Nokia today: Developer Platform 2.0 for Series 90: Designing XHTML/HTML Content v1.0. It supercedes a previous pdf file entitled Developer Platform 2.0 for Series 90: Designing Web Services. The 7700 ships with a customized version of Opera 6. The browser itself is designed to reasonably handle HTML 4.1, XHTML 1.0 and 1.1, WML, XHTML Mobile Profile and others.
The phone will render a normal web page as best as it can in 640×320 or less pixels, but markup written in XHTML-MP and designed for a mobile phone would probably be ideal. Of course, not all HTML-MP phones are created equal. In order to deliver the best user experience, it might be best to sniff user agents and send 3650/6600/Series 60 users to one page and Series 90 users to another.
This could get complex and out of hand quickly, but in many cases it could be worthwhile. Of course you could also target a Series 90 specific site and write the XHTML-MP code optimized for the screen layout. A mobile site tailored to 7700 users for instance, could easily go for a 7700 specific layout. Keep in mind that your users might be coming in at 640×320 in fullscreen mode, or they might have as little as 480×217 free if they are browsing with both the command bar and toolbar shown. I have a feeling that many users will spend a good bit of their time surfing in 480×217 mode or similar, as long as the site permits. This way they have easy access to navigation buttons and it will also keep them in familiar gound with the surrounding UI. Also note that you loose another 20 pixels if there is a scrollbar involved.
So far XHTML-MP has not been covered very thoroughly in the blogosphere, technosphere, or the book world. In the near future I hope to address some of that with some musings on XHTML-MP.