Or alternatively you can extend from mx.controls.Alert class to be able to use your own extended ExtendedAlertForm class from mx.controls.alertClasses.AlertForm class that will control the text alignment in the way you want.
More on this approach here: http://firstname.lastname@example.org/msg37410.html
To align the text, use the textAlign style:
Other possible values are: left, right
I have solution for text alignment in alert popup. You must edit class mx.controls.alertClasses.AlertForm, but first copy this file from flex framework sources to your project folder (like this: project/src/mx/controls/alertClasses/AlertForm.as). You can make it beauty, but I only change newX variable to zero in updateDisplayList function.
// Center the text and icon horizontally and vertically. newX = (unscaledWidth - newWidth) / 2;
newX = 0;