[typoon]

Are they safe to delete? I did a search and couldn't find anything on this. Anyone know?

[wataru]

I think they're only used to determine correct permissions (by Disk Utility's repair permissions function). I usually delete receipts for anything that I move from its default directory, or receipts for random 3rd party stuff that really shouldn't use a .pkg installer.

[himself]

I'd leave them be, just to be on the safe side. I believe that they are required for determining correct permissions, so they wuold need to be there in the event that permissions needed to be repaired. At the very least, leave the Apple software receipts in place, especially those pertaining to system software.

[Rickster]

IIRC, they're also used by Software Update to help determine which updates to install (e.g. whether to offer an update for separate Apple products like Keynote, whether you've already installed a given system-related update via other means, etc.).

Also, some (current and forthcoming) third-party utilities can use receipts for other useful features, like telling you where a given file came from or uninstalling an entire package.

I'd recommend keeping Apple receipts around, at least. It's not like they're wasting much disk space.