Although it is documented in the release notes of the R2 Beta, we have had several comments that the product keys provided with the beta version of R2 do not work when installing on a non-eval version of Windows Server 2003.
Why you ask? The R2 setup re-keys the entire computer with the R2 Product Key..which becomes the new product key for WS03 & R2 moving forward. If we gave out non-eval R2 keys, it would be the same as giving out full products keys for WS03.
NET/NET – Eval versions of WS03 must be used for the beta of R2.
The good news is that by the the time the Eval expires R2 will be RTM'ed!
– Ward Ralston