ok, so looks like this is the case
the iphone carrier bundle file is an xml that pretty much defines the phone's data connections (and a few other things)
with os 3.0 beta, you can add a "type-mask=-2" as a key in the dictionary for the apn that should be used for tethering and mms
however, they've gone an made the assumption that the same apn will be used for both mms and tethering (i'm guessing that's how AT&T's is set up)
on vodacom, this is not the case, the "internet" and "iphone.vodacom.za" apn's do not have access to the vodacom mms gateway
only the mms.vodacom.net apn has access to the mms gateway
so, you can pretty much choose if you want tethering or mms
i've got ipcc files for either if anyone's interested