The only way to do it, as far as I know, is to actually go and edit the "Adobe Illustrator Prefs" file (in Library/Preferences/Adobe Illustrator CS5 Settings/en_US) with TextEdit.
Use search to find this line: "/maxRecentFiles 10". Below it you will see a list of the recent file represented in a bunch of characters.
What a mess, eh? So much for including a simple "Clear Menu" command in the application, as every true Mac app does. Windows users seem to have it worse, as they have to modify the registry for this (
see).
The funny thing is that Photoshop actually has the "Clear recent" option, but neither Illustrator, InDesign nor Fireworks do.