We had separate media with our select agreement.

Since they were pre-keyed, I never paid attention to the keys.

I would expect since SharePoint had 1 media, and the key determined the version, this was not the case with SQL Server.

One thing that comes to mind - since the trials are usually Enterprise, you might want to check that avenue and see if something is noted there.

Or if you deal with MS more directly and have some kind of select agreement, bounce it off your rep.

2005? Personally, 2008R2 was so much more stable.

I sure you have already checked and made absolutely sure you are not using any Enterprise specific features.