Hi,

Google Android applications on the T-Mobile G1 can spawn a telnetd
that gives remote root access to your phone:

http://www.android-unleashed.com/2008/11/howto-get-root-on-your-android-g1-and.html 

This particular method needs user interaction, but a rogue Android app
could easily run telnetd automatically.  Android apps are not normally
granted this sort of permission, and granting root is not supposed to
even be possible.

-jim