It looks like a bug http://code.google.com/p/android/issues/detail?id=939.
Finally I have to write something like this:
<stroke android:width="3dp"
android:color="#555555"
/>
<padding android:left="1dp"
android:top="1dp"
android:right="1dp"
android:bottom="1dp"
/>
<corners android:radius="1dp"
android:bottomRightRadius="2dp" android:bottomLeftRadius="0dp"
android:topLeftRadius="2dp" android:topRightRadius="0dp"/>
I have to specify android:bottomRightRadius=”2dp” for left-bottom rounded corner (another bug here).