How can I install boot camp off a Windows 7 USB flash drive?
I'm having the exact same problem. I tried holding the option key when rebooting to see if I could boot from the USB key but I could not find a way to do this, so I installed rEFIt (then rebooted twice!) and THEN plugged in the USB key and the next time I rebooted it did show up as an option to boot from the USB key. 

I tried that and the windows installer loaded and it found the new bootcamp FAT32 partition. It said I couldn't install to the bootcamp partition because it requires NTFS.

This SuperUser post suggests "simply format the partition using the Windows installer's built-in utility (in other words, as part of the installation process)" so I did that continued with the install and everything works fine!