Lion orphaned installer
I had the same situation. I figure since the OS is installed and working fine, and /lost+found isn’t where it would look for anything anyway, it’s safe to delete that file. Fair warning, that’s an answer based on reasoning, not any official information.

For background on lost+found, I’d suggest reading "What is the purpose of the lost+found folder?" on Unix &amp; Linux SE, rather than the TechRadar article linked in another answer on this question. The TechRadar article wanders, slowly, through several tangentially related topics, whereas the SE answer cuts straight to the point, has technical detail, and has a high vote count from the community.