I ran into this same issue on a new install of mysql 5.5 on a mac. I tried to drop the test schema and got an errno 17 message. errno 17 is the error returned by some posix os functions indicating that a file exists where it should not. In the data directory, I found a strange file ".empty":
sh-3.2# ls -la data/test
total 0
drwxr-xr-x 3 _mysql wheel 102 Apr 15 12:36 .
drwxr-xr-x 11 _mysql wheel 374 Apr 15 12:28 ..
-rw-r--r-- 1 _mysql wheel 0 Mar 31 10:19 .empty
Once I rm'd the .empty file, the drop database command succeeded.
I don't know where the .empty file came from; as noted, this was a new mysql install. Perhaps something went wrong in the install process.