I successfully got Citrix Metaframe Presentation Server 4.0 running with Sql Server 2005. I changed an existing install of Citrix to the new sql server with a copy of the Citrix database. I'm going to try today or tomorrow running the citrix install and pointing to sql 2005. Currently I haven't seen any problems. I've got a profiler running and I'm watching it all and everything looks ok.
All I had to change was the MF20.dsn data source name to the new server and I had to copy the Citrix database. There were some orphaned users, but that was simple to fix and that's it. Cranked right up!
*note: I had to change the sql server name in the registry too. I need to go back and document where that key is at.