Full Review of the T-Mobile G1 Android Device
a) This phone is not aimed at the "average cell phone user".
b) Any "virus" that needs to be downloaded and installed manually isn't going to spread very far. I expect that any security hole that allows a worm to spread will be closed fairly quickly.
c) Symbian, Windows Mobile, and even J2ME already have the possibily of such nefarious applications being developed, but I am not aware of any such instances, despite having had Symbian-based phones for the last ~2.5 years and have recently switched to Windows Mobile.