Well i can assure you that i've used them on my 37" LCD samsung tv and they're more than up to the job, it's been there for about 4 fucking months so i think it'll be ok....
Ok it my not be ideally suited to a 50" plasma but thats your decision, i bought them purely on the description on the pack after having looked at several other options, they are for "Heavy Duty" use, wtf does that tell you.
rog i have no doubt they are heavy duty fixings,but it is impossible to tell how securely the plasterboard has been fixed to the studs or where a joint in the platerboard may be ,which would be a weak point.
they may very well take the weight ,but if you get a kid pulling out of the tv the fixings could loosen over time unknown to you and a accident may be just around the corner.
better safe than sorry imo.
cheers james