BusyMac has a free app: "NUD to iCal" http://www.busymac.com/utils/nudtoical.html I have not used it, but these guys know NUTD and iCal inside out; they were the original developers of NUTD/C. One shortcoming, that they discuss on the above page, is that it does not export recurring events beyond the first one.
There's also $10 shareware app that I have used: "iCal Sync Now Up-To-Date 1.2.2" http://www.macupdate.com/info.php/id/27057 It did export recurring events, but not always perfectly. "iCal Sync" worked fine in Leopard, but does not work in Snow Leopard. I emailed the developer asking if an update was forthcoming, but never got a response.
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