I have shifted by Dev Env recently to Eclipse Helios on Windows 7. After that the Ctrl + Space Content Assist feature is not working. I found out that that key option is utilize
On my Ubuntu, Xfce, ctrl space was allocated to switch language on the iBus preferences (if you have this running, you will see the 'i' icon on the task bar, right click and choose preferences). To change this, bring up the IBus Preferences, on the General tab, see the "Enable or disable:" option, click the ellipses and provide a different keyboard assignment.
This sorted me out without need to restart Eclipse.