Unable to migrate between 10.5 machines
Yes, i'm the IT Manager for an Ad Agency using mostly Macs, and i've done it all, 10.4-10.7 and it's almost never a problem. Are you using AD or something for authentication? Migrating from 10.4 all the way up to 10.7 has always worked for me.

As a suggestion, try installing Applejack on the system and running it. It's an excellent a ppm, and has sorted out issues for me more than a lot of times.. It's been a lifesaver.. Also, Are you using Firewire? or Ethernet to migrate user accounts? Ethernet is by far the fastest/best way to migrate user accounts between 2 machines

Wow, that was totally wrong. Sorry, i was not paying attention when i wrote that...
EDIT: Firewire 800 is the fastest, Firewire 400 is the next fastest, Ethernet is the slowest, and Wireless is unusable.. Sorry... Msg me if you are still having problems, and I'll try to help!