Well, it can be done, but it's a non-intuitive work around.
First, create your slide show in an iPhoto folder the usual way. Then hit the "Export" button on the lower right of the iPhoto screen. From there, you can determine the size of the QuickTime movie you want to make. If it's for the web, I'd suggest the large size.
The movie will render, and then be placed in your iPhoto Slideshows folder inside the Pictures folder. From there, you have a couple of options, none of which are well documented: One way is to open your newly created movie in the latest version of QuickTime. Then from the Share menu, select "MobileMe gallery" from the options presented (you'll get the screen you can see in the thumbnail above). Another way to go is to is to import the movie into iMovie first, and then export it out to MobileMe from there. You can also drag the movie back into iPhoto (and yes, I know this is nutty) and then click the MobileMe icon to upload it there.
Why can't you do this directly from iPhoto? Search me. It should be there -- maybe we'll see it in the app's next version. If you have another method to do this let us know.