NoClassDefFondError in Android... but the Class is in one jar included in the Classpath
you need create a folder named libs in your project, and copy you referenced jars to this folder.
Being unsuccessful at the automatic updating thing, I downloaded the latest and greatest. But the MultipartEntity class couldn't be found. I re-added it to the project. I made sure it was in a folder called 'libs' - but no luck. Andong's prompted me to check the project preferences. Project Properties -> Java Build Path -> Order and Export: make sure that Android Private Libraries is checked. (I checked them all). Clean your project. The problem is gone.
So weird, my app already had the files in "lib" and got this crash. I renamed it "libs" following idiottiger's response and it worked.