I had this problem. Different cause than those mentioned here.
One particular Excel file (named as "Schedules") would open only as "read-only." If I renamed it to something else (e.g. "Schedules 2"), it would open properly as read-write. If I saved it in a different directory with the same name "Schedules", it was fine. On the other hand, if I created a brand new file in the same directory with the same name "Schedules", the new file would be tainted with the same problem.
I eventually ruled out that Excel was the problem, because I tried opening the "Schedules" Excel file with TextEdit, which opened it, but also reported it locked.
I eventually determined that there was a hidden file in that particular directory called "~$Schedules." It was hidden in Finder, but you could see it if you did ls -la in Terminal, or if you searched from Finder for hidden files in that directory. It was apparently causing OS X itself to consider any file called "Schedules" in that directory to be locked. I deleted that and the problem went away.