I was trying to use the built-in screen sharing technology in Mac OS X to connect from my Mac to my Ubuntu machine. I was having the problem where the screen would just sit at the connection dialog and nothing would happen. Well here's how I got it working.
On the Ubuntu Machine:
System > Preferences > Remote Desktop
Make sure the following if set
Check "Allow other users to view your desktop"
Check "Allow other to control your desktop"
Check require the user to enter this password (And enter a password) NOTE: This is the option I didn't check and it was not working without a password.
close the dialog
On the Mac:
Open the finder application
Select the Go menu and select Connect to server...
Server address field put vnc://ip to ubuntu server
Click connect button.
You should be prompted for a password. Enter the password from the Ubuntu settings.
Click ok to the dialog box about support for encryption and you are done.
Hope this help someone and saves them hours of Google searching.