Windows Installer XML (WiX) toolset has released as Open Source on SourceForge.net [from Rob Mensching blog]
I'll play with it tomorrow morning and try to incorporate in our automatic build system ;-)
Update: Rob invented invited a number of the people who helped the toolset for small Windows Installer XML Ship Party . Thank you all for releasing this great toolset =)!
Update (21/04): Mike Gunderloy posted Introducing WiX article for ONDotnet.
Update (31/05): Jeff Callahan posted that soon wix tasks would be available for MSBuild (great! so whole thing could be integrated in VS2005 and commandline build process)