Messaging just became a whole lot easier with the compact QWERTY keypad for the PlayStation 3. Attaches to the top of your PS3 controller for ease and comfort. Push the 'cursor mode key' and, the surface of keypad can also be used as a touchpad just like a mouse ! Also features shortcut buttons to Message Inbox and Text Chat. PS3 controller not included.
I have one (£17 from Play.com but now £26.72 on that site) and it's pretty good. Keys can be a tad fiddly at times but you get used to it. Certainly more tidy than having a USB keyboard sat next to your PS3 all the time and certainly cheaper than most small bluetooth keyboards. (the iGO Stowaway isn't that great IMO).
It also has nice integration with the PS3, like with the friends button and messages button. And the touchpad works quite well.
It has a seperate power supply (doesn't piggyback the controller) so keep in mind you have to charge both seperately. Holds charge for ages though and has an on/off switch so when not in use you aren't draining power.
£22 at shopto, £30 at Amazon, this is certainly the cheapest I've seen it (apart from at launch).
Can also be used on other bluetooth devices such as mobiles or PCs.