There's a fun iPad app called Lisping that let's you play with different flavors of Lisp. Not really a programming environment, but it's sort of neat for trying out ideas.
Edit: reference: http://seattleclouds.com/ticketfiles/8665/ios_program_standa... 3.3.2 "An Application may not download or install executable code. Interpreted code may only be used in an Application if all scripts, code and interpreters are packaged in the Application and not downloaded." So in fact the Lisp app is also forbidden (but of course Apple is free to selectively enforce)