After a pretty long and annoying afternoon, I've found out what the problem was.
I hope this helps someone some day.
You need to specify the unit property for the StackLayoutPanel.
I first tried PX, and thought it didn't work... The issue was that it was actually too small to be seen. EM worked fine:
<g:StackLayoutPanel unit='EM'>
Hugo
2010-01-31 05:12:24