Thanks everyone for your replies. I know that data usage is a more "user" issue than a hardware issue per se, but I guess I'm just not understanding the whole 3G thing from the perspective of, is the phone consistantly sucking data for stuff just while it's sitting there?
I'd like to utilize applications such as GPS for example, I just have no idea how much data that uses when you would want some directions (I know,it would depend on how long you actually keep requesting more GPS info from Google Maps etc).
I think in the end, I'll seriously consider a Bold and try the flex plan. If after a few months, I find I'm not clearing 300MB, then I'll switch down to that plan.
Now I just have to find a way to sync my data flawlessly with my Mac and I'll be a happy guy.
Thanks again everyone!