i've the same problem.
I've well seen that this was generally because of this dll in 32 or 64bits.
But in my case :
- virtual machine window server 2003 (then 32bits)
- visual studio 2008
- wss v3
- Visual studio 2008 extensions for Windows SharePoint Services version 1.2
I 've well taken care of taking the right cablib.dll
And then WSPBuilder / Visual Studio 2008 can't load this dll
Do you see something else ?