This may not work for everyone but it has worked for some and it is useful if you are on 4.3.5 firmware right now and dont want to do the tethered jailbreak.You will need a T-mobile pre-paid sim card that is activated...Borrow one from a friend

Go to the network screen of the iphone (settings-->general--> network), remove the h2o sim, insert a t-mobile prepaid sim, the cellular data network option appears! Opening that tab, reinsert the h2o sim, then fill in the proper settings.

The only problem is if you reboot the phone you may need the tmobile card again to get the cellular data option to return.Otherwise you have to do a tethered jaiblreak for now and install "Tether me" from cydia to get that option...using the source of [Only registered and activated users can see links. ]