I came across this error when trying to create a new list definition based on a content type that i have defined in my project.
I was so confused as to why this suddenly started happening, i thought i may have been when i placed my solution into source control, however after 5 hours of testing, re-writing the code again thinking that my project was broken it suddenly clicked what had happened. Basically you cannot place a SharePoint Project into a Solution Folder. Doing this causes Visual Studio to loose its ability to locate your SPI’s in your project. Removing the project from the folder and back into the root of the solution fixed the problem.
I hope that this helps people save many hours of lost time like i did.
Chris
