I don't know the exact tags supported off hand, but Mobipocket does use a subset of HTML and adds a few of its own tags. Although correct behavior would be to ignore anything that it doesn't understand, it is possible that you just need to clean up your source a bit for it to display correctly.
Here is a link to the custom tags. The links on the left should get you a list of the other supported tags.
http://www.mobipocket.com/dev/articl...agref_mobi.xml