Part of growing up, I guess. A few years back, yes, I would have been gaming, torrenting, all of that stuff, and if I could tether my phone to do it I would have just for fun or because I could. Now, I don't mind paying for music, renting flicks, or a landline internet connection, which was designed to handle this kind of traffic. Not mobile networks.
I do (obviously) have my phone rooted, with a custom ROM. Doesn't affect anybody but me. Trying to use my 3G to substitute my DSL could, however, especially if there was a way to open up ports for various services, which there isn't, and it is most likely done at the provider level, so the only way you could is to get a job with Virgin and break some rules.
It's not about 'underestimating the power of 3G.' Yes, it probably can provide enough speed, although there will be lag issues, I almost guarantee it.
I certainly wasn't telling you what to do, only to consider the reasons why you can't do everything with a mobile connection that you can do with a landline connection.
Also, that OV on AT&T comment just makes me laugh. I hate to be this guy but...."pics or GTFO."