Now that Team Foundation Server 2008 keys/media/downloads are available, I'm seeing lots more questions about various scenarios.  I'm going to try to capture all of the less obvious issue here:

You have TFS 2008 Trial Edition installed and you want to upgrade to TFS Standard Edition

You need to go to Add/Remove programs and click Change/Uninstall on your TFS installation.  This will display a dialog in which you should click on "Upgrade".  You can then enter a new product key.  If you bought full packaged product (and actually got media) you will have the key you need in the material you purchased.  It's not so simple for volume license customers because they don't actually get media - they download the bits from the volume licensing web site and the key is not obvious.  The "official" policy is that you are supposed to call Customer Support and ask for your product key.  There is a simpler way, if you choose.  If you look in your download for a folder called "AT" (stands for Application Tier).  You will find a file called "setup.sdb".  Open that file in notepad (or any text editor).  Search for "[Product Key]?.  The next line is your product key.  You can enter this string into the dialog presented from Add/Remove programs and click OK.  Your TFS server is now fully upgraded to Standard Edition.

You have TFS 2008 Trial Edition installed and you want to upgrade to Workgroup Edition 

You must uninstall trial (your data will not be lost - although backups are always advisable) and install Workgroup Edition.

You have TFS 2008 Workgroup Edition installed and you want to upgrade to TFS Standard Edition

This scenario is intended to work like #1 (upgrade trial to standard), However, there is a bug that makes this a bit tricky.  If you use Add/Remove Programs to enter a new product key, you will find that the product key field is greyed out - this is not "by design".  I've been told that if you run setup from the media (rather than Add/Remove Programs), the ID field will not be read-only.  Alternatively, you can uninstall Workgroup Edition and then install Standard Edition.  As far as getting the proper product key (for volume license users), see # 1 above.

You have TFS Beta 2 or Release Candidate

TFS 2008 Beta 2 and Release Candidates releases are about to expire (the RC expires in the next week or two and the Beta expires in mid March).  Please make sure you upgrade VERY SOON.  You can upgrade to TFS 2008 Trial Edition, Workgroup Edition or Standard Edition.  You will perform the upgrade by unintalling the Beta or RC and then installing your chosen edition.  If they actually expire, you have a 1 time only shot at extending it for 30-days (using the new TFSVersionDetection tool that you will find in this blog post).

You have TFS 2005 and want to upgrade to TFS 2008

 As a general rule, this is straight forward.  You just uninstall TFS 2005 (all your data will be preserved) and install TFS 2008.  New client and old clients will both work.  By far the trickiest part will be upgrading from Sharepoint 2.0 to Sharepoint 3.0 (2007) or MOSS 2007.  My best advice is read the corresponding upgrade instructions (ours and/or WSS).  The TFS installation guide is here: http://www.microsoft.com/downloads/details.aspx?FamilyID=FF12844F-398C-4FE9-8B0D-9E84181D9923&displaylang=en. And here's a reference to the Sharepoint upgrade instructions: http://technet.microsoft.com/en-us/office/sharepointserver/bb421259.aspx

Brian