I think you should read Peters reply again.
The problem is not with the service pack it is with your page execution logic.
You only need to bind (or populate through any means) the DDL on the first page load.
To prevent the data binding again you need to test for a postback, Peter has provide the code for you.
This problem occurs because the page load event fires prior to the Click event of the DDL.
So when you load the drop down list, the first element is selected and it is what the Click event is using.
They say, best men are molded out of faults,
And, for the most, become much more the better
For being a little bad.