If you are having an issue with the assembly version in your compiled assembly not updating after a rebuild then you are not alone. The is a bug in the Visual Basic compiler that causes this problem and it requires a manual fix:
http://kbalertz.com/952628/Assembly-Version-change-Visual-Basic-Workflow-projects.aspx
Not hard, but annoying...
To check the problem, double click on the assembly in your bin folder and open out the version tag.
You can see from the image below what I am getting at. This causes no end of problems with signed assemblies. The KBAlertz article above implies that this only affects Workflow assemblies. But I have has t6his problem in other types of Visual Basic project.
This is not a big problem unless you are creating custom assemblies for SharePoint and have a convoluted deployment process before you can test, and can't figure out why you changes are not going through...