Tm90 is a great camcorder, if you can get it for a good price then go ahead with it. Panasonic v500 is an upgrade for the tm90 , so it should probably have a few improvements and more features but you cannot say for sure unless the reviews are out.
If it was me, i would probably wait till camcorderinfo.com has a full fledged review for the v500, or if its urgent then i will pick up the tm90 and you might probably get a good deal now since most stores would want to clear up their stocks.
Compared to tm90, tm900 is a much better camcorder with better sensors and more pro features. X900 will be the upgrade for tm900 and it would be more or less the same.