First, all ISPs in the US are seriously considering charging their high end customers in the very near future. Running a SHOUTcast server off your home line would most definitely put you in their high end user category (last I remember, if you pushed out over either 1 or 5 gigs a month, they were going to bill you for the additional bandwidth). Remember, these are just talks as of now, but it does sound like something that is going to happen here.
Secondly, it is against the terms of service to run a server on any ISPs home line in the US. However, no one really pays attention to that, so don't be too worried about that one. I'm just saying, technically that is a rule and they can refuse to ever provide you with service again if they so choose.
ATT's elite service caps at 1 meg upstream (if I remember correctly), which means you can only host 8 people at 128k.
Finally, that Comcast speedboost you got really plays with your results --- and in a bad and false way. You don't really have a 3.5 meg upload, sure you do with speedboost, but that speedboost is only going to stick around for something like the first fifteen min. After that it is going to go away on you and you are going to be left with something different.
powerboost = speedboost.