Restart Unity and try again. Restart your computer, if necessary.
If it still happens -- Upgrade to the latest drivers for your graphics card.
answered Dec 27 '10 at 01:52 AM
You need to explain it further..
Do you see a blue shade circle on the terrain? If so, than alter the settings. If not, than maybe try Unity 2.6 and wait until another update comes out.
But I also agree with ina, it probably is your graphics card.
answered Dec 27 '10 at 04:03 AM