update OS from 3.1.3 to 4.0.1...with APN changer?

hi guys!

im new here and i just picked up my first iphone yesterday. Anyways, im a college student and the school has free wifi all throughout the campus and i have wifi at home and so i don't see a need to get a data plan, which is why i loaded the apn changer to the 3.1.3 firmware that my iphone came on. However, when i looked around, i noticed that the 4.0.1 OS was out and was wondering, if i update the firmware to 4.0.1, will i still be able to navigate to this site and load the apn changer to disable the transmission on cellular data so that i won't incur data charges from AT&T?

Also, this might be a stupid question, and i might get flamed for this, but what's jailbreaking? I have AT&T and if i plug my iphone into itunes, it says i can load the new firmware there, so can i just update the firmware on itunes to whatever is the latest, and then come to this site and load the fake APN changer and be good to go?

any help would be appreciated, thanks a lot!