On trying an Android beginner's tute (http://mobile.tutsplus.com/tutorials/android/android-sdk-development/) the "R.id cannot be resolved" error had me stumped too. In my case it was an android:onClick in /res/layout/main.xml. It seems onClick is not supported in AVD platform 1.5 API level 3.
A change to a new AVD with platform 1.6 API, level 6 removed the error - also started the project from scratch with the new api and level settings. For absolute Eclipse newbies (like myself) create a new AVD with:
Window -> Android SDK and AVD Manager -> New. Select platform, level number from "Target" drop down list.
