I wouldn't recommend running SQL Server 2005 on Windows 2008 either, it's frought with issues :w00t:

If you really want to though this is what i'd do (and assuming the hardware is the Windows 2008 R2 HCL).

Pull one of the nodes from the current cluster, flatten the node and fresh install Windows 2008 R2. Install a new sql server 2005 clustered instance (you'll need new storage too) using the same instance name, you'll need to specify a new network name and ip though, these will be temporary as you'll move the existing VIP and VNN across later.

Migrate objects\databases from the old cluster to the new cluster and when ready change the VIP and VNN to use the ones from the old cluster, set the group offline on the old cluster first.

once you're happy flatten the remaining windows 2003 node and fresh install Windows 2008 R2, then introduce into the new cluster