I just recently started getting this error (with the newest version of iPhoto '09), but on only one of my MobileMe galleries.
I started publishing 10 photos at a time from the Event I was trying to post, and found a picture near the end (of course!) that was missing. The thumbnail displayed in iPhoto, from the smallest to the largest zoom in the main screen; but when I double-clicked that picture to see it full screen I got a giant gray circle with an exclamation point in it. I deleted that photo and the gallery published fine!