If you create a skin in Flash CS3 using the Flex Skin Design Extensions, and use a bitmap on any symbol instead of vector graphics, the scale-9 grid won't work.

Steps to reproduce:

1- Install the Flex Skin Design Extensions for Flash CS3
2- On Flash CS3, create a new skin (File > New > Templates > Flex Skins > Button
3- Change the look of the Button replacing the existing vector button with a bitmap button
4- Adjust the scale grid
5- Export the SWC
6- On Flex, use the import artwork feature to import the SWC
7- Add a button to the application and change the width to an high value
8- Confirm that the scale-9 didn't work properly