Scratch runs on Squeak Smalltalk apart from some features that require a Scratch-specific native extension to Squeak. Scratch is not free software, so no wonder distros don't package it. The Scratch guys have a package for the OLPC though.
Here's the best guide I've seen this far for installing Scratch on Ubuntu: http://scratch.mit.edu/forums/viewtopic.php?pid=52179#p52179