Interesting one, this.
Before the merger, Nokia and Siemens both had relationships with picocell suppliers RadioFrame and ip.access respectively. When I spoke to NSN in February in Barcelona, they said they were likely to continue dual-sourcing picos for existing customers, but were still working on their strategy for building/buying femtos.
Given that both their existing partners are also active in the femto area, I hadn't expected them to bring a 3rd option into play - although in retrospect I guess they'd probably have cast their net as wide as possible.
Airvana's femto product stems from its acquisition of UK-based 3Way Networks earlier this year. I'd imagine that, at the time it was up for sale, someone from Nokia might have dropped in to 3Way to see what was happening (given Nokia was presumably on the VC's potential acquirer list), so they may have been pretty familiar as well.
Couple of things - in both the press release & the website, it only mentions UMTS femtos rather than specifically saying HSDPA/HSPA - although I'd imagine that must be on the roadmap. I'm not 100% certain what backhaul mechanism Airvana uses for femtocells - although its architecture diagram mentions IPsec, and I seem to recall its UAG gateway can handle UMA among other options.