Only by tethering to an existing 3G phone. You could do that with jailbroken iPhone, or you could buy the unlimited data plans from either Verizon or Sprint and tether. IIRC, those plans are $60/month, so it would cost you twice as much per month as a iPad with 3G, although you'd be saving $130 one time up front.
They bill you based on total usage (i.e., your carrier data plan), not the number of devices hanging off of it (tethered). The iPhone data plan is unlimited. So, I tether my iPad to it wirelessly when I'm out and about. Have been tethering my MBP to my iPhone for about a year now and haven't seen any increase in my bill. Multiple devices...one payment...nice... BTW, I use MyWi from Cydia for tethering goodness...For those advising the OP to tether his device with an existing phone, I thought wireless companies could see tethered data usage and bill you additionally for using your phone in a tethered state without subscribing to their individual tethering plan?