At least on Vista, my mouse-hating keyboard reflexes have been stymied for a while. (I don't really hate mice, I just hate having to move the mouse when I shouldn't have to. I'm sorry, I'm old. I was born before computer mice were invented, and I developed my computer usage habits long before every PC shipped with a PS/2 port or a mouse.)
With apologies to Bill Cosby, I'm not sure when I lost it; I'm not sure how I lost it. But I know it used to work in Vista... and it still works on Windows Server 2008 in IE7 and Windows XP in IE7. I just tested it on both. [I figured out the root cause while writing this post, but more on that later.]
I've got half a dozen PCs, notebooks and tablets running Vista that I spend most of my time on, and it's really starting to chap my hindquarters when I have to touch the mouse just to back up one field in a web-based form! Inside Microsoft, a lot of things get done (most things?) in web-based forms... and it's always faster to key than to mouse+key.
If this keeps up, I'm going to have to switch to Windows Server 2008 everywhere and get a divorce from the Windows Sidebar (which I love) because it apparently doesn't work on Windows Server 2008, at least on x64, but then neither does many important pieces of Office 2007, such Internet Fax. Double grrr. But nagging teams that don't ship x64 editions of software is another rant for another day.
(Compiling and testing for native x64 isn't hard, people. I realize that it explodes your test matrix by a factor of 2x, but for the love of recycled bits, please budget for that and ship it! I know I said another day, I will!!)
I won't eureka, but I can speculate that focusing on my angst about SHIFT+TAB to type this stream of consciousness probably help me figure out what eeevil possessed my beloved browser and forced it stop responding to SHIFT+TAB on all my Windows Vista machines, but not on my Windows Server 2008 and Window XP boxes... The Windows Live Toolbar, dammit! I never use it except for the Blog This! button anyway (which I can have without the toolbar), so I uninstalled WLT, rebooted and voilá! I have my long-missed SHIFT+TAB back on web pages and forms again. Hooray!