Same boat here.
Once I was ready to do 8 and 16 mans, I was forced to play melee dps because my guild already has a main tank. I left and joined 2 other guilds and same situation. Just no spot for a main tank.
Now I'm a L3 Sith Sorc (yes, I re-rolled last night after being told yet again to spec DPS).
If you are interested in doing endgame raiding (HM FPs is not endgame raiding), the most needed classes are healers and then dps. I'm not even listing tanks here because guilds alreayd have a main tank, a substitute main tank that's currently playing dps, and possibly another standby main tank who's also playing dps.
Check out the same thread I created a while back - http://www.swtor.com/community/showthread.php?t=292867