The temperatures and times involved in powdercoating are not high enough to affect any aluminum or steel frame.

Steel (4130 Cr-Mo) anneals at over 1500° F, and hardening heat treatments soak at close to 1000° F. I think powdercoating ovens are more like the 400° range.

6061 Al anneals at 775° F, and it's possible to precipitation harden at 320°, although it takes 18 hours and then another 8 hours at 350°. For powdercoating, a bike frame would probably be in the oven for 45min or less.

At my work, we send Al out for powdercoat all the time and there is no real concern for the PC process to affect the strength of the material.

*Info was from these links:
http://www.suppliersonline.com/propertypages/4130.asp
http://www.suppliersonline.com/propertypages/6061.asp