Just a short note, looks from the datasheet that the MC34921 drive chip is transistor based. They only list voltage drops for the bridges. The package is rated at 2W dissipation, but the math makes me think it'll be pushing 9W if everything is on full blast. I'll still design the board around that, but it means I can't use it as a RepRap basic SBC.
I'm scaling back to a more basic DC drive (maybe make it a dual chip unit as a bipolar drive) and perhaps combining a MC33486 with dual IRFZ44ZS low side MOSFETs. This should let me drive 10A constant, and 35A intermittent. It's been on my list for a while for other motors. It's FAR bigger than RepRap needs, but this has several advantages. One, I have other projects that I meant a board like this for anyway. Two, this loosens the size requirements for RepRap servo drives, so they can get bigger (anyone want a 1m3 build area?). Three, the heat dissipation with smaller motors may alleviate any need for a heat sink.
Note to self: add links later, too tired right now.