i'm having the same trouble. i bought an unlocked Infuse in the states and i'm using it in canada on Fido (service provider).
whats weird is the plan doesn't work with any app installed on the phone. even basic ones such as youtube and firefox. they all claim i have no internet connection even with my Data plan activated and Wifi disabled. i can't receive pictures or video messages either.
BUT the default android web browser DOES work on the data plan, albeit incredibly slowly. eg it takes updwards of 2 minutes to load google maps, if at all. it's very weird.
i have Use Packet Data checked off.
i tried downloading APN manager and filling in the fields with the data info provided to me by Fido, but still no success. Fido doesn't know how to help me and i've called Samsung, they are no help either. All Fido could do was provide me the following info regarding their data plan:
APN: WAP.fido.ca
RenderIP: 205.151.011.011
WAP Homepage: hxxp://fido.mobile.ca
username: Fido
p.w. fido
(its not actually hxxp written in the phone, just cant write it out fully of this message wont post)
also, within the APN dialogue boxes, under MCC the number 302 shows up, and under MNC i see the number 370.
please somebody help! neither my service provider nor phone manufacturer know how to fix this!!