This is a feature I have had in other text editors, and it would be very useful for all of the editors in MyEclipse. I just spent a few hours tracking down a bug in a CSS file that was due to some errant invisible ASCII characters hiding at the end of the line. If I could set a pref to display non-printable ASCII chars I would be very happy.
I searched the forums but found no reference to this topic, I hope this has not already been discussed.