Windows emulation has grown by leaps and bounds in the last several years. As of now, it is pretty much possible to get any major MMORPG working in WINE, the Linux Windows emulator, if enough people get together and work on making the game compatible. There are so many people playing WoW, for instance, that a way was found to get the game working very well in any Linux environment.
As we speak, there are several native Windows games which can be played on Linux without any serious difficulty. Many of them require a bit of tinkering with your setup to get working, but for some people it's worth a little extra work to be able to play their favorite game without having to load up Windows XP or Vista.
I think it's safe to say that within 5 years' time, pretty much any MMORPG game will be playable with WINE in Linux. Much progress is being made in WINE's operability, on a daily basis, and it's not unreasonable to believe that this possibility is very real, at this point.
Some of the Windows MMORPG's which are currently playable in Linux include World of Warcraft, The Lord of the Rings Online, Dungeons and Dragons Online, and Guild Wars. Last I heard, work was moving along nicely on Warhammer Online as well.