THX certification for the Plasma is not the same as certification for AV receivers. THX is like a standards body that defines minimum required specifications for many different types of equipment.
THX cert for a receiver is going to cover things like minimum power requirements, ability to drive speakers with a specified minimum impedance (I think they use 3.2 Ohms), range of crossover options, bass management capabilities, THX post-processing routines, etc. It also covers non-essential but useful options for ease of use such as the color coding of the speaker outputs.
I don't know exactly what THX cert for video entails but I'd assume it has to do with video characteristics like color space, ability to render blacker than black and whiter than white, supported resolutions, etc.
Having any equipment that is THX certified is useful in that at least you know it meets certain industry-agreed minimum requirements. But as everyone will say, other equipment that is not THX certified is not necessarily inferior.