Ars Technica Reviews OmniWeb 5.0 Beta (read review), but misses something on the tab issue: horizontal space is much less valuable than vertical space, especially in web browsers (and many other text-heavy formats), as almost all screens are wider than they are tall, while text pages tend to be taller than they are wide.
John Siracusa makes many greats points (as expected) about how tabs can be done to maximize user efficiency. His section on OmniWeb's tabs explains a great idea: allow regular tabs AND the new thumbnail/drawer mode. However, there is one additional concept that can improve on this. The regular tabs should still be in the drawer, but when they are only a small number (user-customizable, of course), the names of the tabs run vertically, and the drawer stays very narrow. For example:
|Drawer with vertically-running tabs||Drawer with thumbnails|
Basically, OmniWeb could have some smart defaults, but allow the user to customize the thresholds at which it switches between horizontal versus vertical tabs (but ALWAYS in the drawer - remember that the vertical space above a page is more valuable) and the vertically-running tabs, and the thumbnails.