Hi,
one of my colleagues was developing new custom workflow activities. After updating the assembly, the new activity didn’t show up in the Workflow designer. I found similar blog posts handling the same issue:

http://social.microsoft.com/Forums/en-US/74f35d99-1ce5-404c-b451-c092cd6c2754/crm-2011-custom-workflow-activity-not-available?forum=crmdevelopment

http://crmdm.blogspot.be/2012/07/deployment-of-custom-workflow-activity.html

http://microxrm.blogspot.be/2013/05/custom-workflow-activity-not-appearing.html

Our conclusions:
-IISReset is the hard way.
-Altering the strong key means meant unregistering the assembly and due to the prior dependencies not feasible.

Other solution?
Update the AssemblyVersion and AssemblyFileVersion.

Happy coding!
Philip