When I browse my home network with a bonjour browser, I see this _acp-sync._tcp bonjour service beeing advertised by all my AirPort base stations. Can anyone give me any insights on what this is for?
The only likely reference I can find for an ACP protocol is in the man page for natutil:
natutil uses the ACP protocol to detect, configure, and extract status information from Internet gate-way gateway way devices (IGDs) (such as AirPort Base Stations) on the local network.
I doubt you'll find out much further about it without firing up a packet sniffer as the dearth of information about it is a good indicator that Apple want to keep it proprietary.