Update: I mistakenly said this tool does two-way sync. PhotoCopy currently only moves photos one direction--from iPhoto to Flickr.
Having recently picked up one of the new MacBooks, I've been spending the last few days getting it up to speed with all my Web services and prior storage backups.
One of the biggest hurdles I'm trying to overcome is being too reliant on software, which is considerably easier with today's cloud services.
One that caught my eye on this front is PhotoCopy from 24x7 Digital. This $20 software program, which is Mac-only, hooks up with your iPhoto library and Flickr account, and will keep your Flickr photos synced up with those on your machine--sans any manual uploading on your part. The big idea here is that any changes made on your machine will be quietly handled in the background; this includes rearranging, edits, renaming, and deletion.
This certainly isn't a new idea, but it's a smart one. The latest iteration of Picasa does two-way sync with your Picasa Web albums, which can be a great way to restore a huge photo collection, if your hard drive goes kaput. Hopefully, future versions will do two-way sync, and support Apple's higher-end photo management tool, Aperture, along with other Web photo hosts.
Note: If you were planning to use the PhotoCopy free 30-day trial, it adds a watermark to each synced-up image until you've registered, so be careful.