Jerry R. aka "JewishAMerPrince" had it right. When talking about THX you can be talking about different things.
Most people here are talking about THX certification of equipment. That does indeed mean that "THX certified" equipment meets a minimum series of standards. This certainly means that you are going to be getting decent equipment and not junk. It does not mean, however, that if a piece of equipment is not THX certified that it is junk. There are many great sounding pieces of gear that do not carry the THX certification. As with any stereo equipment purchase...let your ears be your guide NOT some letters on the faceplate!
As Jerry R. points out, however, THX also refers to the "post processing" algorithms that THX certified receivers and pre-amp processors employ. Two of the primary goals of Lucas in creating sound for movie theaters in particular (and home theaters as well) is to (1) create the same sound for every person sitting in the theater (an incredibly difficult undertaking involving numerous sonic compromises) and (2) keeping the viewer's attention focused on the screen. How this is accomplished is (a)to have a disproportionately large portion of the sound come from the center channel and ( to make the side/surround channels sound "diffuse" so as not to draw the viewers attention away from the screen. This also serves to make the sounds "location" cues more generalized so that the sound is more consistent from seat to seat in the theater. Therefore, true "surround" characteristics get muddled and even lost. Because of a much smaller audience seating area, home theater systems do not need to be limited by these kind of THX post processing compromises (this is why THX surround speakers are dipole designs - to generate a "diffuse" sound). Much truer surround fields can be generated in your home (front to rear...left to right) than can be generated in a movie theater. This can create a much more realistic feeling of "being there" and being enveloped in the sound. You really can create the sound of a jet flying over your head. Therefore, IMHO THX post processing for movie theaters is a necessary evil but for home theater is crap. Why make an unnecessary compromise?
J. Gary