The new .Mac Web Galleries sure are pretty slick, but their dependency on iPhoto '08 can have some unfortunate side effects. When I first installed iLife '08, I moved my original iPhoto library so I could tinker around with the new version using a few pics I had lying around on my Mac. I then created a Web Gallery with these images just to see how things looked. After deeming it safe to proceed, I replaced my original library to actually begin using the new iPhoto features - the only problem is that, somehow, the .Mac Web Galleries you create seem to be library-specific. In other words: when I created a gallery with my test library and then swapped it out for my true library, the .Mac Web Gallery listing in iPhoto's sidebar was no longer there. I couldn't find any kind of web admin panel at the .Mac site for managing these galleries either, so I quickly found myself in a predicament.
Fortunately, I'm not alone here, and I found this thread in the Apple Discussions forums that describes a method for deleting a gallery manually if it no longer appears in iPhoto's sidebar. The solution basically involves logging into your iDisk from the web (http://idisk.mac.com/UserName) and browsing to the directory that houses your galleries, then manually deleting it from there. For some odd reason, this folder doesn't appear in your iDisk on your Desktop like your other .Mac website files, even with hidden files turned on (I used the Show-Hide widget to quickly toggle hidden files. As far as I can tell, Apple simply doesn't sync this folder down to your Mac or give you access to it with the iDisk). The only problem I had with the solution as it is described in that forum thread is that I didn't find the _gallery folder where the forum poster said I should; instead, it's in iDisk > Web > Sites. Each gallery you have created is stored in its own directory under that folder, so you should be able to delete just the ones you want without affecting the others.
[Update 2: Matt Ronge, developer of upcoming Mac OS X email client Kiwi, enlightened me in the comments with an even easier solution to this problem: simply go to iPhoto's Web Gallery pane in preferences, and simply click Check Now to cause iPhoto to check your .Mac account for any new photos. This should also cause it to catch any galleries not already listed in iPhoto's sidebar, ultimately allowing you to manage and delete them from the comforts of iPhoto.]
[Update: Some commenters are reporting that the _gallery folder in question does actually appear in their iDisk when viewed in the Finder. I don't know why only some of us see it, but you could be one of the lucky few.]
Considering that your Sites (for the old homepage.mac.com pages) and Web folders are synchronized and fully available in your iDisk, I hope that this strange gallery behavior is simply the result of an oversight. While I understand Apple's desire to use software to manage this stuff instead of forcing users to dig around in folders, having manual access to this directory will be useful in a pinch, especially for users who, for example, buy a new Mac or need to have a sick Mac restored.