This comes on the heals of T-Mobile unveiling plans in March of 2010 to upgrade its national high-speed 3G service to the High Speed Packet Access Plus (HSPA+) technology, which will deliver customers data speeds faster than the current 3G network technology . By the end of 2010, T-Mobile expects to have HSPA+ deployed across the breadth of its 3G footprint, covering more than 100 metropolitan areas and 185 million people.
How ironic, with AT&T, the slowdowns come for free on the iPhone everywhere and it doesn't matter what the bandwidth usage is :).
See related story:
T-Mobile Wi-Fi World Phones