I've had two machines in a workgroup connected via ethernet (through a switch) since installing Vista in early 07. Setup was standard - same workgroup name, simple connection. Haven't had issues at any point until experiencing a power flash from the weather (while away from home).
The only other abnormality to speak of would be the Vista machine's clock being slightly fast (which I've since corrected in the BIOS). I've checked to make sure network discovery is on in Vista. Both machines still have their shared folders available.
Everything was normal prior to coming home and realizing the power had caused each machine to reboot.
Fwiw... problem solved. Went into the BIOS on the XP machine, disabled LAN peripheral, rebooted, shut down, went into the BIOS again, re-enabled LAN peripheral, rebooted... workgroup is showing up now.