I've been using the iphone since november on t-mobile and i recently switched to ATT
and here are my observations and input.
T-mobile:
The 5.99 package doesn't have a limit on data usage as far as i know (never was on for 48 hours straight in a day). With the 5.99 plan some sites are blocked or do not work because of port issues i believe. Some examples are: mobile.msn.com, dynadel.com. You also cannot post replys on certain forums like my350z.com. And you may not search on youtube. I know if you pay for the total internet package on t-mobile, all these problems will go away. You'll be able to visit those sites and search on youtube.
As far as a know the website thing was a recent thing. You were able to go to those pages and post in the forums maybe about a month ago. I guess t-mobile is catching on.
The tzone hack isn't new. If t-mobile cared.. they would stop it on the ENTIRE network and not spend time and resources monitoring individual users.
I guess tethering your iphone to your laptop would be reason enough to get you kicked from the plan because that is not what the 5.99 plan was intended for. I see logic in that.. however what the OP has to say bares none at all.
I could honestly say my tzones experience has actually IMPROVED with time. I get fewer and fewer errors with webpages and increased speed and reliablity. Yes i have had times where edge refused to work for day or two... i waited it out and the issues resolved themselves.. without me having to interfere.
But hey.. those are my two cents.. you know.. from a guy that actually uses the service.