I know for a fact that texting switches the phone to 1x. Again, texts are sent via the control channel on the 1x "Voice" portion of the cell site"
HowStuffWorks "How SMS Works"
Now, that's not to say that RF conditions can't force your phone to switch between 3G and 1x. The EVDO carriers may be programmed with different propagation parameters that can affect the actual coverage. So if you're on the border of coverage your phone will drop 3G first, go to 1x then drop completely.