Screen Sharing (VNC) is working fine but I just wondered is there a way to use it if my iMac is in Sleep mode? Will it do a wake-on-lan type thing or does the iMac always have to be on? Could you then put the iMac to Sleep when you are done on VNC?
There's an option in sys prefs/energy to allow waking from sleep via ethernet, but I don't think it works over airport. Doesn't on mine at the moment anyway
It won't work with airport, as explained above but the option in the energy pref panel let you wake the mac when it's sleeping: the option is called "Wake for Network Administrator Access".
If you use VNC I suggest you try this client, which include an option to wake the host computer when connecting: