Well, I think its just a matter of the amount of pressure a group puts on the feds for finding them. TOR is not 100% percent, iirc there are some rather successfull attacks against it, so once a group like LulzSec starts releasing stuff that is really hot (millitary documents, e.g. US war logs or the nuclear weapon codes :D ) the feds or rather the nsa will think of something new. I guess they have the monetary means to setup a few TOR nodes...