http://android.foxykeep.com/dev/how-to-fix-the-classdefnotfounderror-with-adt-17
Thanks ggp.
Spent a good hour trying to resolve this problem after updating to 20
Quick fix; Rename the lib folder in your projects to 'libs', edit the jPCT lib in Properties->Java Build Path, point it to the jar in the new location 'libs'. Good to go...