After installing ADT 21 and restarting Eclipse, several of my projects now had errors where there were none before the update. The problem and fix were fairly straightforward: all of the source files having problems were referencing generated *.R files (i.e. com.foo.myapp.R), usually from a project library that prior to ADT 21 Eclipse/ADT wasn't able to deal with properly without an import for the file. Simply removing the import statement for the *.R files resolves the errors as Eclipse appears to now be able to properly resolve the referenced ids without it.