As used in the Android 2.1 Camera app, in com.android.camera.ImageManager:

public static boolean hasStorage(boolean requireWriteAccess) {
    String state = Environment.getExternalStorageState();

    if (Environment.MEDIA_MOUNTED.equals(state)) {
        return true;
    } else if (!requireWriteAccess
            && Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
        return true;
    }
    return false;
}