I'd expect a lot of stales. The Pi is a GREAT little device, but I doubt it has enough oomph to handle everything a pool server needs.
For example, bitcoind is going to have to be running in the background staying up to date. The memory requirements alone are kinda pushing it for the Pi, but you're probably good... but, then comes the disk access for syncing the blockchain. The SD card isn't going to take too kindly to that, plus the CPU is going to by busy verifying need blocks and any incoming shares.
All this, BEFORE even adding a pool server.
It'd be cool to try, and would for sure LOVE to hear how it goes, but I (personally) wouldn't give it high odds of working out well.
+1, if you want to run your own pool you'd probably be better off getting something like
this and throwing Linux on it.
