I searched but did not find any hints, so I did some investigation, after a while (finding out how mounting works, reading man autodiskmount and some wild guesses), I came up with this solution. Edit the com.apple.finder.plist file (in your user's Library / Preferences folder) using your favorite text editor. Here is the top part of the file:
<!DOCTYPE plist SYSTEM "file://localhost/System/Library/DTDs/PropertyList.dtd">Insert the following lines below the <dict> tag:
<plist version="0.9">
<dict>
<key>AutoplayAudioCD</key>Keep the rest of the file intact, save your changes, and then logout/login (or relaunch the Finder). Your discs should no longer autoplay.
<false/>
<key>AutoplayDVD</key>
<false/>
[Editor's note: I thought for certain there was a Finder pref for this, but I certainly can't see one! This seems to be the best solution at present ... if there's a simpler way that I'm missing, let us know!]

