Multiple instances of Skype in Windows Vista

Multiple instances of Skype 3.x in Windows Vista won't work because of different Runas behaviour. Skype 4 fixes that issue in version 4, which is in beta, using the /secondary command.

For Skype 3.x, some of the runas functionality does work, when the user specified is not part of an Administrators group, but rather that of Power Users.

More in-depth information here.

L., 04.04.2008. Update:
Skype 4.0 has now been released and it does indeed support the /secondary command when using the Runas command:

runas /user:Mart "\"C:\Program Files\Skype\Phone\Skype.exe\" /secondary"

Note that nested quotes (quotes inside quotes) must be escaped with a backslash, as seen above.