I am just replacing a Linksys NSLU2 running Debian Lenny with a Seagate Dockstar running Debian Squeeze as our home file server. We have only Mac OS X, iOS4, and linux computers. I run netatalk for file sharing, Time Machine backups, and shared iPhoto and iTunes libraries (I use avahi so that the server shows up in the Finder's sidebar as an available afp server on the Macs). I also run the Darwin calendarserver and use its CALDAV server for a shared family calendar in iCal and am working on getting the CARDDAV server working for a shared address book. I have had this basic setup on the NSLU2 reliably for over a year now and am just getting it running on the Dockstar. It works great with our Macs, iPhones, and an iPad. In Debian Squeeze you can easily install these packages with apt-get install netatalk, avahi-daemon, calendarserver.
If you can install and manage a Debian server, you home file sharing possibilities are pretty limitless. For your Pogoplug, you should look at: http://jeff.doozan.com/debian/