The main thing you have to realize about the Rasberry Pi is that it has 1 bus, and that is a USB 2.0 bus. Therefore, your NIC and your drives all will be fighting for bandwidth. The processor in it is running at 700 Mhz, but the config.txt files on the R-Pi allow you to overclock and over-volt it. The PP v2 pink has a 1.2Ghz processor and is designed for this type of use, whereas, the R-Pi's Broadcom processor is designed for multimedia use hence the icluded GPU.
Not saying that the R-Pi won't make a decent low end NAS for $35, but I doubt it will touch the PP v2 pink.
Google is your friend...try it sometime.
http://archlinuxarm.org/platforms/armv5/pogoplug-v2-pinkgrayhttp://www.raspberrypi.org/faqs