Maybe it is the manifestation of same problem described here:
Workaround to Run VNC Server in Windows Vista
The problems are caused by Windows
Vista new security feature called
Session 0 Isolation. Previous versions
of Windows ran system services in the
same login session as the locally
logged-in user (Session 0). In Windows
Vista, Session 0 is now reserved for
these services, and all interactive
logins are done in other sessions,
causing VNC server unable to accept
incoming remote control connection
request.
and the proposed solution:
So in order to make VNC server on
Windows Vista works properly, the
workaround resolution (at least until
the VNC developers to come out with
proper fix or update to address the
new security constraint in Windows
Vista) is to run VNC server in user
mode.
Or try if it works with UltraVNC.
P.S: Here is the MSDN article about Application Compatibility: Session 0 Isolation.