Actually the $45 data plan is not required for exchange or VPN. The $45 plan is for large corporations and businesses that pay for the service. It's something the AT&T corporate rep would set up for the line with your company's account manager/IT for the line. But if you you pay your service directly as an individual, you'll take the $30 personal plan.
Both the $45 and $30 are identical, and $45 is just meant for business accounts (because they use the service more according to AT&T)
Of course if you call ATT and ask, they'll gladly set you up with the $45 plan. Don't do it, it's only for chumps. Most of the CS reps don't know the difference either.
This is no different than how they have been doing data plans for Windows mobile for a couple of years. Since it's integral to a WM device no one questions it. But they do have a $45 for business and $30 for personal just the same.
Exchange can be and is used for personal as well as business use, just like pop and IMAP, so it's not (and can not be) tied to exchange use.