Quite an urgent one this - my solution wouldn't build because a referenced assembly (another project) needed signing with a strong name key. I signed it and it's now "system cannot find the file specified" when trying to deploy!? I've checked windows\assembly and it's not in there.
NOTE: This is when I attempt to deploy a dependent project/feature to SharePoint. It builds fine, but fails on 'Activate Features'.
Sorted it now; I'll post the answer just in case anybody else comes up against the same problem.
For the wsp I'm deploying to also deploy the project dependency - it needs to be included in the project manifest. You get to this by double clicking on Package (in the primary project being deployed) > Advanced tab at the bottom > and then adding the assembly from project output.