I recently ran into the error “Missing Operating System” on a bootcamp MacBook Pro with Win 7 and OS X Lion 10.7
After “Googleing” as much as I could, all the other possible solutions didn’t work for me. Which made me think, “What is different about my setup?” Bootpicker!
A very easy simple step I tried was to open System Preference on the Mac, open the Bootpicker settings and low and behold the “Windows” partition was set to the new Lion Recovery partition.
After modifying this to actually point to the Windows partition, log out and use the bootpicker screen to select Windows, walah!
Hope this helps others, please let me know by clicking on the vote link below!