SharePoint Workflow failed to start

If a workflow is created through a WSPBuilder project, the workflow would have problem to start. To solve this problem, the .csproject file needs to be modified manually. Open the project file in notepad, add the following line

Import Project="$(MSBuildExtensionsPath)\Microsoft\Windows Workflow Foundation\v3.5\Workflow.Targets"

right after the line

Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets"


Then problem solved.

This is really a frustrating issue.