I have successfully downgraded about 20 servers in our environment. I am working on doing the same to the clustered ones too.

I tested cluster downgrade on a test cluster and it worked.

We had SQL Server 2008 Enterprise everywhere and had to downgrade in lower environments to SQL Server 2008 Developer.

The basics are this. Install Developer edition and look at the registry in Setup dir. Take those keys and copy them to Ent. server. You have to restart SQL Service for it to "know" it's now Developer edition.