Failed app installation, storage full, safe to delete files manually?
Before you do anything - make a Nandroid backup using ClockworkMod Recovery or any other recovery tool that you may have.  This is really important as it means that anything you can can be undone if needed.

That said - I believe you can delete these files.  I've not removed files from /data/data/ manually before, but to the best of my knowledge it won't have any bad side effects.  I recommend you try just renaming that folder to something else to start off with and run your phone for a while to see if anything bad happens, if not then you're good to just remove it.