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Sprint's "everything" plans are very competitive, giving you truly unlimited data, voice and texts, with just a $10/month "smartphone tax" which still keeps my bill well under what I was paying at T-Mobile.
The downside is that Sprint doesn't have good coverage everywhere, and their mobile broadband for laptops isn't unlimited like it is on phones. The price for that kind of thing is prohibitively expensive for anyone who doesn't absolutely rely on it for their business.
The great thing about both T-Mobile and Sprint is that they are very relaxed about tethering. If you have an Android or BlackBerry phone with it built in, they don't seem to notice occasional tethering. I've even read (anecdotal) accounts of Sprint employees dropping hints to customers on how to tether without buying the separate plan.
Since I picked up a Nexus S 4G, I've used the built in tethering app without any problems. Granted, I'm not pulling down multi-gigabyte ISOs or anything crazy like that, but it's there when I'm in a pinch and need it.