My darling Mikey,
I love all my nieces and nephews alike -- but you are all my favorites! As for battery boost solutions, there are basically two kinds of iPhone solutions currently available on the market. You can go for cheap and awkward, or for expensive and well-designed.
That's not to see that the cheap and awkward solutions are unmanageable or wrong, it's just that the cheaper solutions tend to be large battery dongles that extend off your iPhone's connector port at the bottom. If you want a battery solution that fits more like a case, just expect to pay more.
I personally own a pair of Monoprice dongles -- the same ones my colleague David Winograd praised as "such a deal." Costing just over $12 for the 2200 mAh, you can recharge your iPhone while on the go. Attach the dongle, which is about half the size of the iPhone itself, for thirty minutes or so and your iPhone will be fully recharged. You can usually get more than one recharge off the pack.
If you're looking to pay a little less, you can pick up a 1900 mAh unit for nine dollars and change. It won't provide the same boost but it will save you a few dollars.
These low end units share two big problems. First, they change the shape and weight of your iPhone when in use. If you're running low on battery and need to use the phone with the booster plugged in, you'll find that the device becomes much more awkward to use. The longer shape makes it particularly awkward for landscape interaction.
The extra weight is definitely noticable. Yes, you can use the phone with it plugged in (and I find myself doing so more often than I care to admit), but it's not as smooth for use.
The second problem lies with iPod touch units. Using a plug-in booster like the Monoprice units will cover the audio out jack. That means you can recharge or use headphones but not both. I've looked around for a male to female connection cable extension but have not yet succeeded in finding one to buy [they're out there -- both dirt cheap and relatively pricey. –Ed.]. Even if I did, it would be seriously awkward carrying around a dangling battery charger pack.
Bottom-attached packs work best when you plug in the device each time you stick it in your backpack, purse, or briefcase. It ensures that your unit recharges while you're not using it and you always pull out a fully-charged device. There are also some chargers designed to work with your regular sync/charge cable, like the well-regarded 4400 mAh Gum Pro pointed out by our commenters; there are even solar-powered chargers like this Dexim unit, which doubles as an iPhone stand.
You can cut out a bit of awkward if you're willing to spend a little more money. The mophie juice pack air provides "the world's thinnest rechargeable battery case" for $80 (with free shipping). It creates a more case-like solution for powering your iPhone, which at least from a geometric point of view, is really nice. Plus, the juice pack air is officially certified by Apple.
The downsides are the cost (obviously) and the lower boost -- mophie does not guarantee that you receive double the battery life time. With the cheaper dongle solutions, you often get well over twice the life. The 2200 mAh I own is often good for a complete second recharge (note that I do not go to full discharge before recharging), giving my iPhone quite an extended battery life.
Regardless of which kind of solution you end up going with, keep in mind: you're going to increase the weight you're carrying around, you're going to have to be responsible for more stuff, and the shape of the iPhone is going to change -- case form factor or not.
If you're the kind of person who uses the iPhone from a pocket, battery boostage may not be the solution for you. But if you're a purse/backpack/briefcase kind of guy, you may want to look into a booster pack.
Love, kisses & snuggles,