Rick, as far as I know, no LCD will support standard MAME/arcade resolutions natively. They can of course interpret the signal and then upscale it, but they won't display it natively. I think the manufacturers claiming this are either confused or outright lying.
And KillBill, you won't find any 4:3 LCD's beyond 21", at least for a sane price. If you want larger than that, you'll have to go Widescreen. You could always get a 27" widescreen and mount it vertically(permanently). I'm sure there's a way to run the monitor in vertical mode and still display 4:3 content letterboxed at the correct ratio. And then think how vertical games will look, spread over the full 27" screen. A 27" inch widescreen is still about 20" high, so in vertical mode that would be virtually the same as a 21" 4:3 screen.