After hacking the AC3D file I've found what it was.
Even though I didn't set autosmooth blender exported with "crease 30" meaning 30 degrees autosmoothing.
AC3D format probably needs this, but if you like me don't want this you can hack the file by and replace "crease 30.000000" with "crease 89.000000". That will do the trick.
You could also try export with autosmoothing on and use 89 degrees.
Blender also added transparency to my materials "trans 1" but it should be "trans 0". Hack that too.
That's also an exporter error for blender 2.49.
In Blender 2.6+ that smoothing error is present also. AC3D format probably forces you to use this.
Just FYI, I like the new 2.5+ Blender.