I mounted a 37" Samsung TV to just plasterboard in my bedroom about 4 months ago. I bought heavy duty fixings that screw into plasterboard and expand out behind them from B&Q, job jobbed. i know it's probably not as heavy as a 50" tv but it can't be far off.
Cheers for that, is yours lcd or plasma? Plasma are a lot heavier, I couldn't lift mine on my own!!!