Thanks Bartsidee - At least someone has looked into it ;-p
As you said, it may be unlikely that Boxee produces their own Spotify app (at least while focused on bug fixes), but would it be possible to create an app by working in tandem with them?
After all, they may be able to include the requisite third party file if they've been provided with it, and they know the functionality it would add. If they're aware of Spotify I'm sure they'd want it on there as much as we do! (they may just not have the resources to focus on it)
Macbook Pro (2009) | OS OSX 10.6.4 | CPU 2.53 GHz Intel Core 2 Duo | Video NVIDIA GeForce 9400M | Version - 0.9.23.15885
Boxee Box (2010) - 18.104.22.16843