We played with multivariate testing at the last company I was at, using an expensive platform built by Optimost (now part of Interwoven):

http://www.interwoven.com/components/pagenext.jsp?topic=PROD...

(We also paid a company that did optimization via Taguchi methods, and found them to be a bit of a scam.)

Besides this and Google Website Optimizer, I'd be curious if anyone on HN has used other AB or multivariate testing tools? I would absolutely love something open source that I could plug into Django templates on the server side, rather than depend on external Javascript code to rejigger the page.