I'll second this request. I'm maintaining legacy code that's 10 years old and it requires tabstops every 8 characters, but an indent width of 4 spaces. Doing the expand, unexpand trick won't work on this old code because the final unexpand introduces tabs into sections of code that didn't have them before.
Unfortunately, I have to stick with Vim when editing this code. Vim's great, but I prefer TextMate's intuitive interface and elegant appearance and integration with OS X. Great job by the way. If TextMate can help me overcome this tab problem I'll gladly purchase a license.
Cheers, Ryan