Android:How to programmatically set an Activity's theme to Theme.Dialog

So I have an Activity (say TestActivity) which needs to act as a normal unthemed Activity as well as a Theme.Dialog at other place. I am trying to reuse same TestActivity for both the tasks.

All I am looking for setting the theme dynamically. The code is simple: Here is my activity's onCreate that works with a black background

public void onCreate(Bundle icicle) {
    if (Utility.isDialog == true)
        setTheme(android.R.style.Theme_Dialog);
    super.onCreate(icicle);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
.....

and here is the Manifest Entry

<activity android:name=".TestActivity"/>

And in the meantime I found a post that says it can't be done here is the post http://code.google.com/p/android/issues/detail?id=4394 .But there is a strong feeling that it can be done.

All suggestions are welcome.