I'm running the latest Parallels 6 build, and just upgraded to OS X Lion, Win 7 VM. Also having the issue with USB drives.
I came across this article 
http://go-dslr.com/reviews/mac-os-x-lion-what-works-and-what-does-not-photography-and-others
And ran the following command, while the VM was running:
pauls-imac:~ paul$ sudo kextutil -t /Library/Parallels/Parallels\ Service.app/Contents/Kexts/10.6/prl_usb_connect.kext/
Password:
Warnings: 
    The booter does not recognize symbolic links; confirm these files/directories aren't needed for startup: 
        /Library/Parallels/Parallels Service.app/Contents/Kexts/10.6/prl_usb_connect.kext/Contents/CodeDirectory
        /Library/Parallels/Parallels Service.app/Contents/Kexts/10.6/prl_usb_connect.kext/Contents/CodeRequirements
        /Library/Parallels/Parallels Service.app/Contents/Kexts/10.6/prl_usb_connect.kext/Contents/CodeResources
        /Library/Parallels/Parallels Service.app/Contents/Kexts/10.6/prl_usb_connect.kext/Contents/CodeSignature
Suddenly, magically, I am able to attach the USB drives to the VM. 
I didn't have a problem with anything else, so I just ran the USB related fix.
I'm confident that this could be in the next update, its a pain to have to do it every time I start a VM manually.
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