I have tethering and MMS working. I didn't use the method you described but just stuck with the .ipcc that I've had since beta 1. As soon as I stuck it on my 3GS, tethering was working but I had to
call AT&T several times to get them to provision my account again. Unfortunately when you upgrade a phone it re-sets your account. Unlike with the betas, it seems that ATT cust service and tech support are a little less willing to enable mms. I wasn't taking no for an answer and eventually the tech support guy caved in.
The funny thing is, every AT&T person I spoke with kept trying to blame apple for the issue and one guy even went on a tirade of how apple charges $99/year to develop for the iPhone. I was pretty dumbfounded.