#$@#%.... apparently, this is normal in cs4 according to the help files:
'When you change the frame rate, the new frame rate becomes the default for new documents.'
This is indeed weird. Remember this window where you can change the stage size, background color AND fps? Which has a 'Make Default' button? So what's the point of that now, since the fps is the only setting that doesn't stay default? Don't know why adobe did change this, but I could do without it...
Also I've found out that when you change the ftp in the timeline window, it turns back into the default fps when opening a new document.
When you change the fps in the property inspector window, it stays that change fps when opening a new document.
A bit weird, but at least I can change the fps now without turning it into the new default fps: by changing it in the timeline window.