A KB article has been published on the usage of .net 3.5 with blackpearl. Essentially K2 blackpearl is compatible FW 3.5 and related service packs such as SP1. So, you can install it on the K2 server and K2 client machines running K2 Visio components or the VS.NET components.
But, since All blackpearl releases including 807 are compiled on FW 3.0 they are not aware of 3.5. So, when you want to develop components using FW 3.5 and use it with K2..
1) if you are using FW 3.5 based client components that also initiate K2 process instances, interact with tasks or smartobjects using the provided K2 APIs (which are FW 3.0 based) you should not face any issues.
2) If you are writing server events or other server components for use with K2 server that uses FW3.5 then you need to thoroughly test it.