Can I use the theme uint test data in a demonstration site for a commercial theme? Or am I only allowed to use it for internal development?
Everything on the Codex is GPL (or can be considered as GPL-compatible). Insofar as any part of the Theme Unit Test data are copyrightable, unless stated otherwise you may assume that the WordPress Foundation holds the copyright, and that the work is released under the GPL.
Feel free to use it however you see fit!