It seems the B model of the Pi has been updated to 512Mb memory. Hopefully the Pi that I have on back-order will turn up with this as the one I have only has 256Mb.
I've been doing a bit of searching to see if anyone has a compiled Seti app. It seems a few people do but none are in the repo. In fact if you do a sudo apt-get install boinc-app-seti it returns an error saying it can't find it but its referenced.
The other issue is there is an armel version and the Pi should use the armhf (ARM hard float) version so the apps need to be compiled for this platform to get the best performance.
The good news is you can download it manually from the Debian pakages page:
I had to download it to my PC, stick it on a USB memory stick and then copy it across to the Pi. Once its there you'll need to unpack the files into the appropiate directories. The File Manager that comes bundled with the Pi will allow this.