It is definitely an issue with CPU count not being powers of 2, it was only picked up when the 6-core CPUs started to arrive on the scene and was fixed in higher service packs of SQL 2005 and 2008.

I went to a SQLBits conference here in the UK around Easter and a few of the DBAs presenting mentioned this issue, both with Cluster and Standalone installations.

Setting the CPU count to a factor of 2 will get round the installation issue. Microsoft seem to be a bit reluctant to issue media with the service packs included.