I used to have one of these keyboards for my old HP iPAQ hx2400 Series Pocket PC. I loved this device as it gave me the chance to type up long notes, work on documentation etc. How ever the smart phone arrived and as it was this was the end to my iPaq (the iPaq was great at the time).
Once I got my first iPhone 3G I thought I would be able to use the keyboard with this, great I thought, however I was very disappointed when it said that the iPhone at the time would not support bluetooth keyboards.
As like most tech people I did not dispose of the keyboard, it goes in to the boxes in the loft, you never know when you will need it again. Most tech people I know have boxes of old crap somewhere which is full of stuff you know you will never need again, like parallel printer cables, dial up modems and serial cables etc, but we just have to keep it.
From there I left it and never went back to it again, but on Friday when I was attending a Microsoft Skydrive Event, one of the tech bloggers there pulled out this little keyboard and starting typing away with her iPhone 4. Currently I am still rocking a 3GS (until March, when I am due an upgrade), so I wondered if it would work with my iPhone.
So as soon as I got home I looked in my database of all my tech kit (no it's not sad, it's organised, thanks Bento), found the keyboard straight away, plugged in new batteries and boom straight away it connected and I could type away with my phone, happy days. Looking back bluetooth keyboard support was enabled in iOS 4, so it has been supported for a long period of time and many times I could of resurrected the keyboard.
Now I can carry this very small lightweight keyboard around with me in my bag, so in the event I am out the office for a day and forget my iPad or I am not near a machine, if I need to, I still have the ability to type away happily on a proper keyboard straight onto my phone. Despite its portable size it is a full size keyboard which I find comfortable to use and can still type at quite a pace.
Below are a couple of photos, so you see the rough size
iPhone 3GS and Stowaway Keyboard 
iPhone 3GS Docked
I am always interested in your thoughts so if you have any comments or feedback then please feel free to add any comments, or you can mail me here.