Either a matched pair of condensers in an X/Y inside the piano or a stereo mic. Beyond that, it's pretty much wide open.
As for myself, I just got a VP-88 for just that purpose that I'm already totally in love with compared to the Realistic electret condenser I used before (dial 1-800-self-noise). eBay, about a hundred off the going rate... not as cheap as I would have liked, but well worth it, IMHO.
The VP-88 has much more believable stereo imaging than many of the less expensive stereo mics (like the X/Y stereo shotguns...). It has the ability to send mid and side instead of matrixed stereo, and it can be either phantom or battery powered, so you can use it for field recording where you don't have a powered mixer handy. I figure a good chunk of the time I'll have it hooked up to my AG-DVC15 (or, as I like to call it, the beast camcorder from hell

).
Now, if only it came with something longer than that frigging 2 foot cable. The cables barely reach to the edge of my studio grand, about four feet short of the mic cables I've semi-permanently installed under the thing.... Oh well. Off to hack some cables together.