Really? I've heard that powdercoating rims isn't a good idea, because "rims flex more than you'd think" and it tend to flake off...? No, I don't remember who I'm quoting, there. Dang memory.
Here's a thread on a local BMW forum with some recommendations... http://www.bmwpugetsound.com/vbb/showthread.php?t=38706
Dependable Wheel Repair in Renton repaired some wheels for me, they do a great job. http://www.dependablewheel.com/