While iPhoto only supports photocasting via .Mac, it can subscribe to any photocast. Since photocasts are basically RSS feeds containing pictures, it's possible to subscribe to already existing feeds. To subscribe, use iPhoto -> File -> Subscribe to Photocast, and enter the feed URL there.
Try these, for instance (Control-click on the links and choose Copy Link from the pop-up menu, then paste into iPhoto):
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060112092821456