Your phone is just acting as a wireless router, simple as that. Put a password on the wireless signal as you would a router and all is fine.
No one can send anything to your phone.
The wifi version saves you on price and 3g subscriptions but not on the convenience of just being on 3g in a flash. If you don't need to be on all the time, then the wifi version is the way to go.
Which carrier are you on? As long as you stay within your plan's data package, I see no issue with occasional tethering. It's when you abuse it by letting the whole dorm leech on for hours or torrenting that 4gb Justin Bieber concert etc, that it becomes an issue.
Sent from my SPH-D710
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