Get a 3-in-1 iPhone case for $19.99
If you're planning to clip your iPhone to your belt anyway, why not do so with a holster that doubles as a battery pack? And triples as a desktop stand?
The Grabit 3-in-1 is a belt-clip case. It's a battery pack. And it's a kickstand for easy iPhone viewing. (It may also make julienne fries--I'll have to double-check on that.)
Geeks.com has the Grabit 3-in-1 iPhone holster for $19.99, the lowest price I've seen anywhere.
Alas, shipping will run you a fairly ridiculous $9, but you can save a couple bucks by applying coupon code GEEKBERT at checkout.
The Grabit features a swiveling, spring-loaded belt clip that can pop out to hold your iPhone upright on a flat surface. (It should also do well at horizontal positioning for movies and the like.)
The real trick up the Grabit's sleeve is its 1,050-mAh battery pack, which effectively doubles your iPhone's battery life.
The pack doubles as a rubberized, quick-release holster--one simple press of the upper clip and you can pull the iPhone free.
(If you want to see more and better photos, check out this listing on eBay--where the Grabit is selling for $29.98, plus $5 shipping.)
The only real downside I can see is that the Grabit keeps the iPhone facing outward, making it more susceptible to accidental damage. On the other hand, an inward-facing design would eliminate the kickstand advantages.
Also, there's no mention of iPhone 3GS compatibility, though I'm not sure why. Most other iPhone/iPhone 3G-compatible battery packs also support the 3GS. My guess is this one would work, though it's just that: a guess. (If anyone knows for sure, please say so in the comments!)
This isn't the only product of its kind. Case-Mate offers a similar battery extender/belt-clip case, but it sells for $79.99--and doesn't pull kickstand duty.
For about $27 out the door (with shipping and the aforementioned coupon code), this seems like a pretty sweet accessory for iPhone owners. I'm seriously considering getting one myself.