The latest version of VNC Session Manager now includes support for choosing your existing session to connect to (requires zenity for auto-detect to enable the dialog). It now also works with home directories exported over nfs. There's a patched binary for TurboVNC too.
The port to RealVNC 4.0 is finished, now you can use VNC Session Manager with the latest version of RealVNC!
VNC Session Manager adds the nessary glue code between VNC's two execution modes that enables xdm based login to disconnected sessions. i.e. it provides a 'Terminal Services' like login for *nix based vnc servers.