So I took a chance (a $25 chance, to be exact) and bought a "mini Bluetooth keyboard" for my Xperia. I figured why not - it may be an improvement over the soft keyboard if I'm doing a lot of entry or something.
These devices are sold everywhere under different names. The one I bought is from Focalprice. Others have the same unit for similar prices.
Naturally, the unit that came was packaged differently than the unit shown. There was no manual, and the included CD was either blank (likely) or wasn't formatted properly for a standard PC to read it. Figures.
After some digging, here's how I got it to work:
Grab the Teksoft Bluetooth keyboard driver for Android, called BlueInput. Get it here. These are demo versions, so they're free to try to see if they'll work with your keyboard or not.
(I recommend getting all 3 versions of the driver. The latest 1.7 driver doesn't install on some devices, including mine. The initial 1.5 driver worked but didn't map the symbols correctly, so none of the keyboard symbols worked for me. I ended up on 1.6.131, which did the job for me.)
Install the driver using your favorite app manager.
Pair the keyboard by charging it, turning it on and pressing the only button on the front of the keyboard. The blue LED will blink. Discover the keyboard on your phone. Type "1234" into your phone, and "1234<enter>" on the keyboard to pair.
Launch the BlueInput app, and follow the instructions available from Teksoft or from here. Follow them carefully, and it should work.
If it works, you'll need to register BlueInput for 10 euro with Teksoft. The demo versions all have some kind of limitation on them.
It looks like the 1.7 driver has support for different keyboard mappings. Note that symbols did not work on my device with 1.5, but did work on 1.6. If they don't work on your device, you may need 1.7 so you can use a different emulation map.
After goofing around with it for a bit, my impression of the keyboard is: not bad at all. Takes some getting used to, as the keys are spaced bigger than the virtual keys on the soft keyboard. It's still a thumb keyboard, and don't expect to be able to touch-type on it.
The keys are rubber, not hard, and have considerable travel - or what feels like considerable travel after using an on-screen keyboard for a while. I was semi-expecting a hard 'chiclet' keyboard, but that is not the case here. Typing is slow, and until you get used to the spacing, expect some errors from pressing the wrong keys.
Navigation and other commands can be done with key combinations that are reasonably straightforward. You can do most things with the keyboard that you can do on screen. I don't know if it's actually faster or better than the on-screen, but if you really hate the virtual keyboard it might feel like a step up.
Physically, the keyboard is nice looking, about the same size as most phones, and very light. It would be easy to pack along. And it's cheap, so you can't really complain.
I'm not sure I'll use it on a regular basis. It does fit in between the virtual keyboard and a full-size in terms of functionality, but don't expect it to solve large data entry problems. The backspace and some symbol keys are oddly placed, so expect it to take some practice before getting good with it.
Note the keyboard has no auto-repeat. If you want to do a lot of text editing, you may want to use something else that supports auto-repeat.
It also seems as though the driver makes you re-pair and/or re-connect every time you turn the keyboard off and on. Re-pairing might be a limitation of the demo, I don't know. Reconnecting is a necessary function of the driver, and they make it reasonably straightforward, but don't expect to turn it on and just go. You need to scan for the keyboard and sync up before using it.
Finally, the keyboard lost the connection to my phone once. It may have been a timeout thing - I put it down for a few minutes. Reconnecting solved the problem, but again, it was a bit odd.
Hope this helps someone interested in the accessory keyboard for their Android.