This seems to be a bug, the problem is affecting VNC remote desktop and TeamViewer remote where if the display is "SLEEPING" teamviewer doesn't connect to it, and VNC doesn't even see the machine being active.
APPLE Please check this issue, it was working before with Leopard and Snow Leopard, but it doesn't with LION.
I use TeamViewer and VNC a lot on my machines, and the only crazy work around is to turn Display Sleep mode OFF, thankfully ScreenSaver does work properly.
Display Sleep should turn your display off, but not affect your machine, as hard drive is active.
Thanks.