Tough choice for me. I've never really gone over 500mb/month, averaging about 300mb/month this year, using the phone whenever I want without ever considering data usage. So I'm inclined to save the $5/month, but out of principle I almost want to have the option of "unlimited" just in case. I'm really not sure how a faster phone (processor-wise) will change my browsing habits. Half the time I might have used my 3G I just get too frustrated at how long it takes to load and how cumbersome it is. With a faster processor and more RAM, offering a smoother experience, it's possible my data usage will go up. And with true multitasking, maybe even more streaming of audio and stuff. I can't imagine I will ever go over 1GB/month though (don't plan on ever tethering with the current options).
What I don't get is how you get 2GB/month but need to pay more money just to have the ability to tether. And then it just shares the same 2GB limit?! Why not just give us the sme 2GB/month and "free" tethering so we can use that 2GB however we please. If we go over (and you might/probably will with tethering) then you pay more. Seems like a win for AT&T to make more this way (by people paying $10/gb over) vs offering expensive tethering that almost no one will pay for.