MSN adCenter is having signup period today, starting at 12 (EST) or 9am (PST). Before you sign up, or after you sign up, you may want to know what to expect from the program. Werty, a moderator at WebmasterWorld, started a thread just for that, named MSN AdCenter Primer for New Users. He lists off many tips and concepts you should be aware of, including what to expect, how adCenter differs from AdWords, known issues and much more.
For more information check out WebmasterWorld.