Yesterday, after rooting around the web for a solution and then chatting with Fabienne a bit about porting a version of the app to OS X, I tripped across FlickrBackup. Sure, it doesn't have all the bells and whistles that Fabienne wanted, like preserving tags, groups, comments, notes, etc. from Flickr, but it does one thing and does it well: it downloads all the pics you've previously uploaded to Flickr to a central location on your computer, to a zipped archive, or to a CD or DVD backup. Of the over 1600 pictures I just downloaded from Flickr, only about 12 of them didn't come down the pipeline properly, instead resulting in a blank image with a Flickr logo and a note reading "This image temporarily unavailable." This would seem to be a Flickr issue, rather than a problem with FlickrBackup.
All in all, not too shabby. It's especially good if you've just got a new MacBook Pro and you wanted to grab just your favorite pictures for carrying around rather than sifting through all the pictures archived in iPhoto and other photo-management applications on all your other Macs. ;-)
For other cool things Flickr related, make sure you check out this page.
Update: Use this tool at your own risk! So it did a great job of downloading all those pictures, but it also somehow managed to delete several of my recent pictures. Not cool.