for polymod's eclair:
go to Settings>Wireless & networks>Mobile networks>Access Point Names> then select your APN and then press the menu button...delete the apn. after that it will not connect again even after reboots.
if you find you need to use data again (in case of emergency...)
just go back to Access Point Names and select 'Reset to Default'. in the menu.
I have a "Donate" button...for those of you that feel compelled to do so.
However, I don't feel I deserve it. Nor will I guarantee any results in return from any such donations.
Although I do appreciate the gesture, a simple thanks will suffice.