Well, here' the deal.

Jailbreak phone gets to;

1) Add lot's of themes to phone from lockscreen to homescreen. See winterboard thread. Super cun.
2) Disable Edge connection and other connections in the phone.
3) Lot's of 3rd party apps to use and mostly free and good. Example..
a) BiteSMS to forward sms and contacts which is not available without Jailbreaking...yahoo.
b) OfflineMaps.
c) Video recorder like Cycorder.
d) Snapture, one of the best camera apps. better than default iphone camera app.
e) PDAnet to share 3G connection with laptop.
and more...............