I'm typing this on the Targus model, available at Staples and Radio Shack. At first glance, it looks like it might have been designed by Apple. It uses the same 'Chiklet' style short-travel keys, and it reminds me of the old black MacBook. The size is similar to the Apple keyboard but the black body and keys help repel or at least hide the dirt that shows up on the white keys of the Apple model. Designed for iOS but also Mac devices, it sports an Apple Command key as well.
There are other 'bonus' keys that make life easier for use with an iPad. The arrow keys make it easy to edit word processing documents. The top left button performs the same function as touching the circular Home button on the iOS device. Next to that is a search button that takes you to the Spotlight Search screen. A dedicated lock button in the top right toggles the lock screen on the iPad! A keyboard button toggles the on screen keyboard on or off. There's even a slideshow button that launches the built-in photo slideshow. It works like a 'Boss Button' to hide whatever you might be doing on the iPad! There are mute, volume up and volume down buttons, plus play/pause, rewind, fast forward buttons. There are rumors that the AppleTV already has Bluetooth built-in and it may be activated soon. If so, this could be an ideal remote control keyboard! The Targus keyboard is lightweight thanks to its plastic materials, compared to Apple's almost indulgent reliant on aluminum. The weight helps when traveling, but it feels a bit cheap and the keyboard bounces around when typing. Although it can be used for Macs or even PCs, I would recommend a heavier duty keyboard for those systems, as long typing sessions could be a nuisance. I also tried IOGear Multi-Link Mini BT Keyboard. It's a lot smaller and matches the width of the iPad in landscape mode. The keys are smaller, too, but most of the key shrinkage is done by reducing the size of the non-alphabetic keys, like the usually traditionally oversized Caps Lock, Shift, Tab, Enter and similar keys. Since those keys are used less often, it's a good tradeoff if you want a miniature keyboard, about 2/3rds the size of the Targus or Apple keyboards. It also works with a PC and includes a dedicated Windows button along with volume, media and traditional function keys.
The IOGear keyboard is very light and extremely portable. However, with physical keys roughly the same size as the iPad's on screen model, you don't gain a lot in terms of touch typing ability. But it works with an iPhone or other Bluetooth enabled smartphone, so in that regard, it's a step up for a small device. If you insist on physical keys and want to keep your iPad screen dedicated to your app, without the onscreen virtual keyboard, the IOGear model is an option. It's certainly small enough to throw in your gadget bag and go! But the smallest option I tried was Verbatim's aptly named Wireless Bluetooth Mobile Keyboard. What sets this keyboard apart is that it folds in half for travel. It is very portable... pocketable even. In fact it comes with a leather-like case. There are mobile friendly keys, as well, like a Home button and media controls. It even works with Android and the BlackBerry Playbook. There is a lot to like with the Verbatim keyboard and typing on it is actually similar to many laptop keyboards, with a few exceptions. Because it folds in half, the space bar is really two smaller space bars. Non-alpha keys are smaller than normal, including the number keys. Pairing the Bluetooth device requires pressing an indented button, so a pen or toothpick-sized stylus is required.
If you're looking for an extremely portable keyboard, for iOS and/or Android, the Verbatim model is my recommendation. If you can want to be able to touch type on the road, and share a keyboard with a Mac and iOS device, the Targus model is my recommendation.