The TFS 2008 product keys have just been shipped and as I've just gone through the procedure of upgrading our trial version install. As there has been quite a few questions floating around about it. I thought I'd point out this excellent post by Martin Woodward on the topic of upgrading a TFS 2008 trial install to the full standard version.
He goes through the whole procedure step by step with screen shots and all and then points you in the direction of Brian Harry's VersionDetection tool so that you can make sure everything worked as it should.
I must say that I as many other have had my doubts about if this would work or not, when the day came to upgrade from the trial version, however after going through and upgrading our TFS install yesterday, I'm happy to say it worked perfectly. This was very nice to see after all the various problems that was associated with installing/upgrading TFS 2005.
Well done Microsoft and the Visual Studio/TFS Team!