Well, I have made a complete of the Raspberry Pi (256Mb version) for the french magazine Linux Pratique. I have tested a lot of distributions, for me at the moment of the test, XBian was the best for multimedia using with XBMC. At first because XBian uses a swap partition and not a swap file. The second reason is that the SOC is overclocked by default. I think that the last version of OpenElec overclocks the SOC since the setting of dynamic frequencies is allowed by the Raspberry Pi foundation (preserved warranty).
One thing also interesting is the possibility to use the remote control of the TV to navigate into the interface of XBMC.
I use it to read films in 1080p from my ftp server... and everything works fine!
The interface of XBMC is not so fast as my htpc via ION video processor, but it's a good solution for a very low price.

For the development, you have a good distribution named Occidentalis from adafruit:

http://learn.adafruit.com/adafruit-r...talis-v0-dot-2

This distribution supports I2C protocol and hardware SPI.