There isn't yet, I looked. However, if you dial *#*#4636#*#* (or *#*#Info#*#*) you'll get a menu where if you click phone information, you can switch from CDMA auto (PRL) which is 2g/3g to just CDMA which is 2g, or Ev-do, which is just 3g.
The closest trick I have to making a widget is if you download anycut from the market, then add it as a shortcut on your homescreen of choice, and when it gives you the chance to select what you want it to do, select activity and scroll down to "phone info". Now you have a shortcut to those options on your homescreen. Not exactly elegant, but it's served my needs pretty well.