What I have done here is use my Sprint Mogul tethered to a Cradlepoint CTR350 and powered the router with a battery pack I made from Radio Shack parts.
On the Mogul I went to Start>Programs>Internet Sharing>USB connection. Since both the Mogul and the Cradlepoint have 192.168.0.1 as the default IP you must change one or the other. It is much simpler to go into the router settings and change its IP. I used 192.168.10.1 for the Cradlepoint and all works fine.
The router will run 3-4 hours on rechargeables. Any cell phone that has EVDO will work fine as long as its compatible with the router.
Of course if you have access to wall power you can recharge the phone and run the router at the same time without worry.
Going this route you can "fly under the radar" and not worry about 5GB limits as long as your cell plan is unlimited data ..and you're not crazy- stupid with downloading. You do not need a PDA (phone as modem) add-on for your account.
I made this because I already had a CTR350. Cradlepoint does make a portable router that comes with an internal battery - the PHS300
There is a whole list of compatible phones on the Cradlepoint Support site.
So, if your air card goes kaput ...use your phone.
