I ran into a fairly irritating filename length limitation with .mac yesterday.
I was uploading some pictures to my Sites directory and attempting to view them in Safari. Most of the pictures worked except for one that had a longer name than the others. I could access the file from my iDisk but Safari could not see it. I shortened the name of the file and suddenly it was accessible.
It appears that .mac has a 31 character filename limitation for web served files. Maybe they're using OS 9 servers?
I created a test page to show the problem using images with name lengths from 24-40:
http://homepage.mac.com/awalker/lengthtest/
All of the images on that page should display properly but I don't see anything past "1234567890123456789A1234567.gif".