Compatability error Installing Sharepoint w/ SP1 on Windows 2008 R2 RC

You will need to install it with a slipstreamed copy of SP2 which has just been released & provides support for R2:

Q: Will Windows Server 2008 SP2 and WIndows Server 2008 R2 be supported? A: Yes, they will be supported on their release

If you are just using WSS there is a link in the blog article for a donwload for Slipstreamed builds. If you are using MOSS you will need to create the slipstream yourself because:

Q: Will there be a MOSS slipstream build? A: So far there is no plan to create a MOSS slipstream build. Customers can refer to the slipstream guide on TechNet mentioned earlier in the blog to create their own build based on their needs.

A Microsoft guide to create the slipstream for MOSS is here.

Another guide on making a slipstream image/install is here.

Edit: Added more details about slipstreaming and MOSS/WSS versions.

I know this answer is quite late for you, but I came across this question when I had the same problem myself and wrote this blog as a step by step instruction on how to slipstream install Search Server Express.