When I bought my iMac, I did a restore from my MacBook TimeMachine backup. When I continued working on my current project, I did notice a difference between the files that were on my MacBook and the files on my iMac. The files restored from the backup were not up-to-date and were from a few days ago.
I tried this snippet on MacOSXHints to compare Time Machine backups to the original drive, and it found differences between the original disk and the backup.