Dell just announced its new WQHD monitor: the 27-inch UltraSharp U2711. With a native 2,560 x 1,440 resolution, the screen should compete with a 27-inch iMac, though you'll need a dual-link DVI adapter for this one and Apple's Display-Port to dual-link DVI adapter is very bad.
The screen goes on sale next month for a MSRP of $1049, but will generally go for less as all Dell screens do.