You have probably damaged some of the non-reflective coating on one
of the surfaces and are looking at a replacement cost of a few hundred
dollars. Just out of curiosity, what does the owner's manual say
about screen cleaning?

Beowulf wrote:
I cleaned my 60" big screen tv (Samsung DLP) screen yesterday with tissue
and glass cleaner, now a black triangle about 5 inches high has appeared
in the lower left corner, very annoying, looks like perhaps some of the
cleaner somehow leaked behind the screen, damaged something or whatever. I
am depressed about this of course. Is there any way I can fix this myself,
or even pay a TV repair person to fix it? How serious of damage, what sort
of damage, might I have done? Any feedback or help greatly appreciated.