Do you currently have at&t? If so you can upgrade to 4.0.1 (latest apple firmware) and jailbreak by just going to jailbreakme.com on your iphone. To unlock just go to Cydia and install ultrasn0w.
If you have t-mobile you can do all this but you will need to borrow someone's at&t sim for a little while just so you can go to jailbreakme.com and install the unlock, after that you can put the t-mobile sim back in.
To tether you can install MyWi from Cydia (costs money) or use PDAnet.
'01 Bullitt #0054