ext3: too many links!
Apparently when using ext2 or ext3 there's a limit on the number of subdirectories you can create within a directory. This is a hardcoded number and seems to be set to about 215 ~= 32k.
This is related to the maximum number of (hard/soft?)links that can created, as every subdirectory needs a link back to their parent (..). If you run into a "too many links" error with really large folders, you'll know why.. The only way you can change this is by changing the number and re-compiling, which is not worth doing IMHO.
I thought I read somewhere the number of files in one directory is by default around 64k, so I just figured it would be the same for subdirectories.. Guess I'll need to re-organize a bit =)