I've been watching some of the internal discussion aliases over the last week as folks outside of our team have been installing Team Foundation and there are a couple of issues that have been seen that probably deserve some broad communication.
1) You must install WSS in server farm mode - the installation steps are clearly described in the installation documents but are different to Beta 2, so I think it might catch a lot of people out. You might see an error along the lines of "Windows Sharepoint Services is not configured as recommended by Team Foundation Server". If you get this, go back to the install docs and make sure you followed the instructions.
2) Passwords containing "|" characters cause setup to fail. TFS Beta 3 will fail to install correctly if the TFSSERVICE or TFSREPORT account IDS or Passwords contain any of the following characters: double-quote ("); space ( ); pipe (|); less than (<). The results vary depending on the character used, but the message you receive will not indicate that the failure was due to the presence of one or more of these characters in the user ID or password.
One cool thing that we added very recently in Beta 3 to our setup experience is "Find it Fix it" - commonly known as FiFi. This performs a diagnostic on your environment before proceeding with installation and can help catch a lot of common pre-req configuration issues.