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I don't know if AT&T offers this, but carriers in Canada have the option of blocking all data to your account if you just call them up to ask.
Just give them a shout and see if that's an option for you, though you may come across some service reps who will try to con you into signing up for a data plan saying how you'll need it and blah blah blah.
In that case I think that all you have to worry about is the data syncing.
Just shut off auto-sync and you should be fine.
I don't understand why you'd buy a G1 and then not get a data plan for it. 75% of the reason to own the phone is the mobile internet capabilities it gives you.
I think it has already been proved and well documented on here that just shutting off data syncing does not stop the phone from using data. I think there is an application on the market that was developed after this issue was found that does something with the APN so that it can't sync even when it tries to. Have a search around the site as I'm sure all the information is here