iSync has been removed from Lion, the latest version of Mac OS X. And the iSync Plugin Maker is missing, too.
This is bad news to Nokia phone owners. The only known workaround is to re-install iSync from a (Time Machine or other) backup. If you happen to have one.
Looking for better solutions, I'll keep you posted.
Update, 2011-07-22: Google can be used as a proxy, for syncing between Nokia and Lion. This article explains how to set up Lion, while Google explains how to set up your Nokia phone.
Update, 2011-08-23: The workaround above is no good if one already has an Exchange account, and the phone only supports one. That's exactly my case, as my company is using Exchange, and I am using a Symbian phone. Dead end? I thought so until I found out that the Nokia N950 (and N9) does allow multiple Exchange accounts, hurray!
So the real fix is: Get a Meego phone!