Yesterday i reported of the availability of the new ISA 2004 service pack 3.
Today i noticed a new message on the ISA server prodcut team blog about a problem with the SP3 rollback feature.
To read the full story go to : http://blogs.technet.com/isablog/archive/2007/05/03/isa-server-2004-service-pack-3-rollback-failure-and-temporary-resolution.aspx
In short it has to do with the fact that the update may fail when the management console was left open during upgrade.
at least if you happen to receive such an error don't neglect it, but call pss instead. They should be able to help you out.