Not correct. T-Mobile includes mobile hotspot/tethering with all their current postpaid plans. And even though T-Mobile made no announcement, they activated tethering at no additional cost for prepaid plans a few months ago as well.
It's an iOS 4 or 5 thing. Can't remember which one exactly. However, it's only allowed on plans with personal hotspot support (e.g. More Everything, Mobile Share, $30 add-on for UDP, tiered data 4+GB, etc).Have ATT and one change I noticed with using it as a hotspot is that my kids iPods can join it now. Not sure when that changed, is that a 8.0 thing?