Shimano XT Wheels

Yo

I remember reading about the Shimano XTR tubeless wheels (superlight, and beautiful with the 16-spoke design), and how they're ONLY compatible with Shimano XTR disc brakes.

Shimano just put out an XT wheelset, same characteristics as the XTRs just not as light. But nobody's said nuthin' on brake compatibility.

anybody know if they'd work with Avid mechanical discs? Or are they again limited to Shimanos?

I can't say for sure, but knowing Shimano, I'd have to guess that they are only compatible with Shimano - and only compatible with XT, not XTR (just like their crank/bottom bracket combos).