If you are developing your assemblies and SP Solution in the same Visual Studio project (see Andrew Connell's great blog posting for more information) then it is important to remember to to set the SP Solution project to depend on the other projects in the solution otherwise it may build the solution file with old DLLs (last build before the currently executing build).

I shall post up a better description of the problem at some point and I also intend to write up an extended version of Andrew's blog posting based on how I develop SharePoint customisations.