Doug mentioned that Opera7 has trouble setting a width for an absolutely positioned ul -- and he's right. Since there is no width specified, the list wraps instead of displaying inline like it does on every other standards-compliant browser. Applying a white-space: nowrap; to the #nav is ignored by Opera (a bug?), and while setting a pixel width fixes the problem, it creates additional issues in other browsers. The mini tabs work fine on SimpleBits because I'm not using position: absolute;.
Frustrating. But Inc.com gets approximately 0.00001% of its traffic through Opera. Sure, it sounds like I'm giving up on it -- but I'm not interested in finding a hack that is specific to Opera only right now, when it appears to ignore the white-space rule. The rest of the site renders just fine as well.
I'd be interested to hear from others who have run into similar problems with Opera.