Is it possible to setup failover clustering between a 32 bit instance and a 64 bit instance? I am attempting to setup a couple of virtual machines in my lab for testing failover clustering on SQL Server 2008 and Windows Server 2008.

One of the hosts I have is not capable of running a 64 bit virtual machine, however the other host is.  Sure enough, I installed the 64 bit machine (host A) first without realizing host B cannot run a 64 bit VM.

Can I run failover clustering with one node on x64 and another node on x86?  Or do I have to re-install the OS on host A to be 32-bit?
Windows Server 2008 (no R2) did ship a 32-bit variant of the OS (last one MS shipped that way). So you can cluster it. You can install a 32-bit FCI on a 32-bit node. You cannot install a 32-bit FCI on a 64-bit node. So no, what you want is not possible. If you need a 32-bit FCI, install it on a 32-bit OS.