Currently Xvnc does not support MIT-SCREEN-SAVER extension. As a result, programs that care how long the user was idle do not work as good under Xvnc as they do under XFree. For example, in licq the "AutoAway/AutoNA" functionality does not work under Xvnc.
I've tried using the XFree86v4.2-based vnc server (see http://sourceforge.net/projects/xf4vnc/) and it does have the MIT-SCREEN-SAVER extension (as well as many others missing in the original Xvnc). I have no idea whether it's stable enough to be used as a primary Xvnc, but may be it's something worth considering? After all, 8.0 cycle (if it's not too late in the cycle) seems like a good time for these kind of switches ;-)