URLS with Ampersands
MORE input validation. Check all characters. No spaces/whitespace allowed.
Set referer of original, ummmm...
Check for valid URL? Can't be done. Ok, maybe with HTTP_HOST part and http://en.wikipedia.org/wiki/List_of_Internet_TLDs
What happens if two URLS generate the same md5sum? http://it.slashdot.org/it/05/11/15/2037232.shtml?tid=172&tid=93&tid=228
Write all in PHP
Checkout nanourl package in unstable