When I exported a wpmu 1.3 blog and imported it with attachments into a different site running wpmu 2.6, everything seemed to work ok except for the fact that wpmu put some extra characters at at the top of all the jpg files thus causing them to show up as blank place holders. A co-worker of mine wrote a script to clean the jpgs but I'm not sure if anyone know what caused the problem. Let me know if there is any other information you need. I was using ff3.