It seems that every six months or so, a new graphics card comes out that makes all current cards obsolete (slight exaggeration). But all of that extra power is being used to push 1080p or 2560x1600 at slightly higher frame rates. TV technology tends to push towards larger sizes. It seems natural to me to think there would be some demand for an extremely large, extremely high resolution monitor with middling frame rates. Not everyone is a gamer. Wouldn't photographers/graphic design professionals find a use for, say, a 42", 8MP, IPS display? (This would be approximately 3600x2250. At roughly 100ppi, this is not higher density than existing 30" IPS monitors.)
Such a monitor wouldn't even have to be that expensive. If a 30", 100ppi, IPS monitor goes for roughly $1000, then this hypothetical 42" monitor could be priced around $3000 and still expect to sell to a small market of professionals.
There's probably some natural upper limit to the reasonable size of a monitor (unlike a TV), since you sit so close to it. But I'm sitting in front of dual 23" monitors, and I could go for some more space. So I don't understand why monitors stop at 30".
This didn't come out of nowhere, by the way. I was reading this Tech Report
blurb about a GTX 550 Ti 4GB graphics card. They point out that the 4GB of memory are silly on a graphics card that can only play games acceptably at a resolution of 1680x1050. But those 4GB wouldn't go to waste if you had an extremely high resolution monitor and didn't care about gaming.