Been using Aggregate Device on the Mac for a bit now to enable headphone monitoring of tracks recording via a MadiFace ExpressCard. For those that use the built-in output on a Mac you know that it consists of three separate switched outputs - speakers, line out, and digital out, auto-switching based on what’s plugged in.
If you’re recording in Nuendo and have the built-in output assigned to a stereo buss and to the CR, then plug in the headphones, the Mac resets the output to line out and lets CoreAudio know something about a change. Apparently CoreAudio then tells Nuendo there’s been a change which causes Nuendo to hiccup on the timeline. On this particular machine the audio files are contiguous but it appears to drop a second or two of audio on all tracks.
I haven’t had time to check whether it’s unique to the new 2011 MBP or whether it occurs on all the Macs in which we have madi cards installed. The workaround will probably be one of two - either keep a short hp extension cable plugged in to avoid the switching, or get a small usb interface for headphone out.