I began using the Exeltech XP-125 in mid-2001 for precisely this application. I've been running a primary rig off of AC reliably for years using a 33aH SLA cell through a Furman conditioner. The combination is extraordinarily restrictive for a Manhattanite like me without a car, however, and I'm unable to walk with a full rig more than 50 - 60 meters at a time without taking a break.
I have run an Apogee Trak2 essentially since its release for two-track (and as a clock master for several of my surround recordings), but am in the process of upgrading to a Daniel Weiss unit that will be powered in a similar fashion. After having listened to some new traditional portable pieces used by some of you folks, however, the benefit is admittedly marginal (though to my ears still worth the heft) and I'm building a DC-powered front end for the 722 that I'll likely run more frequently.
AC in the field is a legitimate option, but one that can at times be more trouble than it's worth.