Windows 7 entry missing from grub after 12.04 installation
Solution 1:
It turned out that one of the two entries in the existing menu was actually an entry for Windows 7, so the solution was to try both of the entries (while being careful not to accidentally start any actual Windows recovery operations). I'm writing this as an answer so that it can be accepted, but I'd also recommend filing a bug report about the mislabeled Windows entry by running ubuntu-bug os-prober.