What does the new button on the power control widget do? There are 6 buttons on my power control widget:

WiFi, Bluetooth, GPS, ???, Data sync, Screen brightness.
What does the 4th one do?  

P.S. I'm using an Orange San Francisco (ZTE Blade) with the 'Japanese Jellyfish RLS6' custom FroYo ROM, but I've seen this power widget in other ROMS.
It's either a mobile network data enabled switch or a 2G/3G switch, try it and check your data connection.

Edit: got confirmation, see comments.