Obviously a Plasma is going to be heavier than an LCD, but a friend of mine mounted his 37" LCD to the plaster board wall of his new build house.
I'm thinking of doing the same and as a result was interested (given that I know the extent of his DIY skills) in how he managed to ensure it didn't fall back off at a later date.
In his particular case he knew that the beams holding plasterboard walls are spaced 600mm apart and was able to use a stud finder to locate two beams.
He tells me he marked the beams before mounting a solid piece of wood (most likely just over 600mm wide by 1-2" deep and big enough to take all of the bolts for the mount) between the two (using heavy duty fixings). He then used more heavy duty fixings to attach the mount for his LCD to this piece of wood, before finally putting the TV in place.
Now I'm not advocating that you do the same but this is my round about way of agreeing with Gerrard. Do not pay someone (specialist installer or not) over the odds to mount your screen. Instead arm yourself with a little knowledge and get a local building contractor (or DIY savvy friend) to carry out the work which should be relatively inexpensive.
Another option would be to get hold of a copy of the Dummies guide to installing LCD / Plasma Mounts (I joke not). I saw it earlier today in my local Maplin Store, the packs (one for LCD and a different one for Plasma) were over £100 but came with the mount and a book detailing the installation process.
Again I'm not saying buy this book and Do It Yourself but it may be a cost effective way to get hold of the required mount and also some bedtime reading to help get clued up (and as a result not ripped off).