The good: The new cap is 10GB, a fairly reasonably amount for mobile data (really, that's like over 300MB a day). After you hit 10GB, your connection will be throttled down to 50kbps, just like T-Mobile's original cap plan. The bad: It will cover every phone, including the SideKick. Worse for SideKick users, their monthly data is going up to match the G1's. The supposed T-Mobile employee revealing this info also mentions "$34.99 for all you can eat (cough 10GB cap)" though it's not quite clear what he's referring to specifically.
Really, G1 users are most likely to be affected, since it's the only phone T-Mobile's got conducive to really moving that kind of data. How much data are you guys using a month on average, anyway? [Android Community Forums via Android Community]