Maxis may be doing that to try and block the previously available unlimited 3G Maxis users using their non-Maxis 3G iPhone from well,.....using their 3G service unlimitedly. This way they can easily track everyone using their 3G service, and:-

A)in the case of the current new subscribers with the iPhone plan, monitor their usage and charge them accordingly to what the data ceiling their plan allows

B)in the case of non-iPhone plan users using their 3G service on non-Maxis iPhone, track their usage pattern, and *gulp* throttle them back? I hope this does not happen, but crazier things had happened before.

Understand that with the official launch, you've just added thousands of new user, adding more strain on the already bucking-over 3G infrastructure/bandwidth, and one of the way to make sure it does not get overloaded is to limite the data ceiling (500MB-3GB as per the iPhone plans), or throttling. If Maxis does enforce the latter, expect a lot of angry subscribers coming back at them, with their supposed "unlimited" service gets the backburner in favor of the new iPhone plan subscribers.

Or they could change the "unlimited" plan to a specified ceiling, and offer "unlimited" to corporate/enterprise customers utilising their 3G simcards with their 3G data modems only.

Just take a look at the huge uproar happening with AT&T during the few months after launch, and users finding themselves locked out of their 3G service because of congestions. You can easily Google about subsribers threatening, and some already filing lawsuit because their subscription to the 3G service is not as advertised.