2 identical PCs - can I swap a single hard drive between and expect Windows 7/XP to work?

This is perfectly OK. We do it frequently.

Some points to bear in mind:

• Only swap when the OS is really shutdown. Not hibernated !
• If you installed ANY software during the last session (including hotfixes/security patches) give the computer an extra reboot, before shutting it down. This is to make sure all updates are really committed to the registry.
• You can use different USB devices (Mouse, keyboard, and so on) as these are intended to be swapped anyway.
• If the network-card allows setting your own mac-address it is a very good idea to set it to the same value on BOTH systems. Use the value of one of the adapters.
• Minor differences in hardware (like a different CDROM drive, or a slightly different video-card as long as it uses the same drivers as the other one) are usually no problem. Motherboard chipset and CPU should be exactly identical. This prevents you from loosing the Windows product activation.
• Microsoft frowns upon the practice, but you will only need 1 license for the OS.

In general: After swapping let the PC boot once while keeping it off the LAN. You don't want the PC to pull Windows Updates or any software phoning home to check product activation at that point.
Any hardware differences will be resolved by the OS.
Then reboot and reconnect LAN and you are good to go.

Windows 7 is more lenient than XP in this situation anyway. (More build-in drivers.)
Also: Windows in general seems not to throw a fit if you visualize physical hardware to a VM. The other way around can be quite problematic, especially it the target hardware is different than the original hardware from which the VM was build (or the VM was build as such and never existed as physical hardware).

if the pc's are exactly the same, say apart from the serial number, then yes, you can expect it to work. I've swapped drives with XP and 7 between identical or even very similar pc's a number of times and it was always successfull. In fact it works so good that for critical setups on one job I used to order an extra pc so that we could just swap it around should something break.

edit thinking about it, Windows is not even that bad in using the same installations on different hardware: using hardware profiles, I used the same install to boot from actual harware, as well as from a virtual machine with raw disk access running on linux on the same pc. Same trick worked as well with Windows on a Mac: dual boot, but same install worked under Parallels as well. And virtualized hardware can be quite different from the real underlying hardware.

I've done this successfully with XP Professional for years with no problems - I have 3 identical PCs in different locations and simply take the harddrive, in a caddy, between locations.

But I recently upgraded to Windows 7 on 3 new, identical PCs - and Windows is warning me that it needs activating "today", but so far it has kept working, for about 5 weeks.