Miguel announced that Novell just released the source for iFolder and its underlying metadata store (Simias). That shows the power of mono to enable cross platform scenarios based on .Net.
See the Novell Forge project page for iFolder. I look forward to reading their source :-)
One thing that seems a tad limited is the P2P aspect, since it apparently requires that you be on the same network. C# implementation of JXTA anyone?
Update: Slashdot's report.