The Ubuntu Technical Board voted yesterday to ship Ubuntu 7.10 ("Gutsy") with Compiz enabled by default. Although Compiz has been featured in Ubuntu 7.10 Tribe prerelesases, the board has had difficulty determining whether or not it is reliable and functionally complete enough to warrant inclusion in the final release.
http://arstechnica.com/journals/linux.ars/2007/09/12/ubuntu-technical-board-votes-on-compiz-for-ubuntu-7-10