i just discovered (after searching the whole day), with Ableton Live 8/9 and VST Plugins (tested on MAC/32)
isInputChannelStereoPair( int ) and isOutputChannelStereoPair( int )
and the plugin has 8 input/output channels
that ableton live, only clears the first 4 channel buffers (and reuses the other 4 channel buffers without clearing them)
Other hosts seems to work okay, so i assume its a ableton bug, but you never know....