I've successfully integrated SP1 Beta into my Windows installations and they work, i used RT7 Lite 2.2.0 Beta, and i think it is worth the effort to slipstream the service pack even though it's a beta product, cause it expires on June 30th, 2011, plenty of time too test and report anything wrong to MS. Of course, i won't be staying on this build until then, cause i will update too the latest build (RC and then RTM) when MS releases them.