Not really. You can force a logoff using Win32_OperatingSystem's Win32Shutdown() method, but you can't force a logon. Logon is handled by a privileged, secure process and is pretty much explicitly designed to not allow what you're trying, because that'd be a great way to hack a computer.
You could set the registry keys for autologon and then force a reboot.
Obviously it's not production worthy, you don't want plain text passwords in the registry and you'll have to clean up the entries afterwards to prevent it logging in with that user every time you reboot. However, in a pinch, it'll do what you want:
Heh... "you could do this, but only crazy people do it in production!"
Good point Matt; I'd kind of assumed AutoLogon wasn't the goal. But that's been in Windows forever and certainly works, albeit involving clear-text passwords in the least-secure database possible. And you can't make this switch users using the Fast User Switching functionality, but you could force a logoff after setting the AutoLogon, which would be similar.