Here’s what you need to do :
1) Buy a Raspberry Pi (see “Where to buy” in the column to the right of this page).
2) Put the standard OS (“Raspbian Wheezy”) on your SD card (downloads and directions to setup instructions available on this page).
3) Start your Pi, configure it, whatever suits your tastes and needs (that’s not in the scope of this post). Please note that the login is “Pi”, and the password “raspberry”.
4) in the Terminal, type : “sudo apt-get install puredata”, hit Enter (no password required), say yes whenever the terminal asks for it.
5) And that’s it. You can run Pd from the Application menu, in the Sound/Video (or something close) category. It uses Alsa by default and works fine with it.
The overall performance is quite low though…
Edit : I forgot : you should also follow this blog if you want more information on this topic : Limina.log .
Edit 2 : You can download a Raspbian image with Pd pre-installed + some presentations by Miller Puckette + his free book here : http://pd-la.info/pdpi/ I haven’t tested it myself so I can’t tell you how it compares with the standard Raspbian image with Pd installed manually.