ugh - sorry to hear of the negative equity situation; sounds like you're between a rock and a hard place, w/ the $$ worries and the unreliability of the car.
if i were spending ~$21k OTD and needed a car w/ rebates to offset the neg. equity.....i'd say that an SVT focus wouldn't be a bad choice at all. it's definitely one you'd have to commit to for the long haul due to depreciation and the equity problem, but it sounds like that's not a problem.
i very seriously considered an FSVT before i bought my miata last fall - coulda found barely used '02's w/ under 6-8k miles for about $13-14k at the time, and a test drive of a new one was very pleasing. didn't like the feel of the shifter - it took me forever to find first gear in that thing for whatever reason - but other than that it's a blast. the euro-pkg recaros are very sweet, though even the non-euro seats are great too.
you might also consider the new '05 focus ST - not sure about rebates, but the 2.3L mazda engine is a pretty nice piece....not quite as aggressive styling-wise as the FSVT, but still looks pretty good.
i personally would look seriously at the SRT-4, but there's probably not enough price flexibility to help you out - and plus there are little things that bother me about the car (like the manual rear windows and no cruise control, among others).
i'd suggest the scion tC, but that's a car w/ a fixed MSRP and no rebates....so the negative equity situation presents a problem there.
you don't seem too enthralled w/ the GTI suggestion, but that (and the jetta 1.8t GLI) are also cars i'd consider - should be able to get a pretty good deal on one, and the interior is very nice, the turbo engine is pretty stout, and it has a lot of mod potential for whatever that's worth. VW reliability isn't necessarily the top of the heap, though...
i wouldn't rule out mazdas, either - might be able to swing a good deal on a 3s - 4-door or 5-door - or perhaps even a 6, since the rebates there are pretty impressive right now. if you don't need practicality, i'd earnestly suggest a miata - leftover models can be had for $6-7k off sticker from what i see on miata.net!!! can't beat the fun-to-drive factor w/ a miata, that's for sure.
my .02...