I remember the Yahoo guys saying they would open source PNUTS/sherpa at the very first NoSQL meetup. It's a shame that it's been so long and they have not followed through on that.
I'd wager that the real issue is dependency on some of the core Yahoo libraries: there is a whole number of them and they're often so widely used that lot of ex-Yahoo engineers are surprised to discover they're not open-source. The reason they're not open source is that some of these would not be useful outside of Yahoo and create maintenance problems e.g., features that were once missing in FreeBSD, are available in recent versions of FreeBSD as well as Linux but had to be ported forward to preserve API compatibility.