While from what I can see on t-Mobile's site, it does appear that they don't charge for tethering, but I'd still confirm with them. I ran up a $35 bill just testing tethering (AT&T) - connect, browse to Google, disconnect a few times. Luckily, I was doing this FOR work, and they covered the expense, but it can get very expensive very fast (I'm talking hundreds of dollars - even thousands), but it's your choice.