This is a question that comes up quite often. The short answer is: No, not in TFS 2008.
However, it’s a little more complex than that.
The application tier will not install on a 64-bit operating system due to a block in the installer. And even if you did manage to bypass the installation block and get it installed, it still wouldn’t work. It’s not just at the IIS level that we’re stuck with 32-bit, that is just the top of several layers.
The real solution is a 64-bit version of the TFS application tier and that’s what’s in the works for the next version of TFS – codename Rosario.
If you run a dual-tier installation of TFS, then you can use a 64bit installation of SQL Server 2005 or 2008 running on a 64bit version of Windows 2003 or Windows 2008 for your data tier. The application tier would be a separate 32-bit server.
Another option is to put SQL x64 on Windows x64 box, put a 32-bit VM on it, put the AT on the VM and target the physical machine as the DT. Care is required to tune the VM for performance, but several customers have used this scenario successfully. Being able to back up and potentially move the AT VM has some additional benefit.