HyperJuice offers a portable battery pack that you can haul around, plug into any wall socket, and use to charge two items at once (even two iPads at once!) with its dual USB ports. Shipping in a variety of eye-catching colors, as well as the basic black you see in the image at the top of this post, HyperJuice retails for US$129.95 (10.4Ah) and $159.95 (15.6Ah).
I was able to test out one of the lower capacity units and can report that it seems to work as advertised. Port access was just fine and the items charged as promised. I only had two complaints about the unit.
First, it weighs a bit -- about the weight of an iPad. It's perfect for bringing along in a car, or maybe a briefcase, but it weighed down my backpack mostly because I had to bring it and the iPad along together. If you have ever carried two iPads in your backpack, you know that it really starts to drag after a while. And, before you guys who carry MacBook Pros around start pointing out that you're the real men of portable computing, I grant you the point. Just be aware, we're not talking about something on the level of portable keychain charger here, either in price or lightness.
My second complaint was about the plug. Yes, it slid in and out beautifully to provide on-demand charging, but unless you have a vertical wall port to plug into, it's wobbly. I tried charging it in a normal power strip and it didn't go well. The unit flopped around. Vertical charging stabilizes the unit against a wall, without the flopping issues.
I'm unable to relate how well the charger works over the long term in a preliminary hands-on although I'd imagine it functions like any other external battery pack.
I really did like several things about this unit. First, that it can handle the monster demands from not just one iPad recharging but two! That's pretty awesome. Second, the product felt nicely made, especially the shiny metal encasing. The white plastic bits looked as if they might scuff a bit over time, but they're solid enough that it seems like you'll be able to clean them without too much trouble.
All in all, this isn't a cheap product to purchase, but it delivers the kind of power-to-go that many of us on-the-go iOS users are looking for.