[yellowapple]

Bookmarks (AFAIK) don't store the state of input fields and such on a page, while tabs do (though they probably don't if you're unloading them on-the-fly). So, if you're jumping between, say, multiple pages with multiline text fields (perhaps when authoring forum/blog posts, for example), support for lots of tabs is handy in that case.

Also, bookmarks aren't quite as ephemeral as tabs, so there are different workflow implications.