Best I can come up with is an article from ESPN:
http://espn.go.com/mlb/s/transanctionsprimer.html
"There are two Disabled Lists, the 15-day and 60-day.
The only real difference between them is that players on the 60-day DL -- sometimes called the Emergency DL --
don't count against a team's 40-man roster."
Seems if time spent on the 60 day didn't count as service time it would be a "real" difference.