you've obviously heard these:
http://www.royerlabs.com/democd.html
my friend has a pair of 121s and uses them primarily for drum overheads and some acoustic guitar tracks.
i love how they sound on the piano. i'm not sold on them as heavy guitar mics. i like the 57 blended with the 609 and if you want more resolution run the 609 into a nice pre.
on a live sound tangent, i have struggled like mad for literally YEARS to come up with a consistent and feedback resistant method of micing our steinway concert grand piano.
my current system---
1-akg 451 condenser, wrapped in a towel with element hovering 2 inches out of towel, and placed in the treble side of the piano, diaphram over the 3rd sound hole, aimed towards the high register hammers.
this position provides good gain before feedback for the high frequencies and a natural overall sound if eq'ed properly.
1-57 on a boom stand reaching into the far end of the piano, furthest from the keys, set at a 120 degree angle aiming through the bass strings towards the soundboard and angled towards the player.
this position allows the bass frequencies to be more prominent, particularly with the aid of some slight proximity effect, and takes advantage of the better noise rejection properties of the 57.
granted you need to eq heavily on the channel to normalize and provide monitors with strong level, but it works.
the best turnkey solution i've seen and heard is the earthworks dual piano mic system, but that badboy is 3 large, and dedicated exclusively to piano duty.