Like pskink said, you can use an 9patch drawable as the divider. To convert your bitmap to a 9patch, use the draw9patch utility in the tools directory in your Android SDK. Drag your bitmap into the draw9patch app window, then draw a black line on top of the stretchable area (on the topmost pixel line). Once done, save the bitmap as 9 patch (e.g. your_divider.9.png) and use it normally as a drawable for your ListView:
<ListView
android:divider="@drawable/your_divider"
...
/>
More info on 9-patches available here: http://developer.android.com/guide/topics/graphics/2d-graphics.html#nine-patch