Running AVRStudio 32 and JTAGICE MKII on Vista 64
Personally I use a virtual machine running XP...

I notice that Dean Camera (a regular over at AVRFreaks) has been playing with getting AVR tools to work on x64 boxes. See this posting on his blog: http://fourwalledcubicle.com/blog/archives/467. He also mentions that he's recently installed a beta of AVR Studio (http://www.atmel.no/beta_ware) which supports Windows 7 and x64.