With Windows Vista we told ISVs "Hey - applications may break on Vista. You should test your application".
What some ISVs appear to have heard is "Hey - other ISVs application may break on Vista. They should test their application".
I am going to try an alternative approach this time (tongue in cheek).
"Hey you. Yep - I'm talking to you. No - not that guy next to you. I mean you. Yep, you Mr ISV developer. Turn off your ipod. Let go of the mouse. Now read the following very carefully and very slowly.
Y o u r a p p l i c a t i o n m a y b r e a k o n L o n g h o r n.
Y o u s h o u l d t e s t i t.
B e f o r e y o u r c u s t o m e r s d o.
And the good news is we have the guidance you need over at http://devreadiness.org, more specifically in the App Compat Cookbook.