Bret,
Here's code that works for me;
DODEFAULT()
THIS.SelStart = LEN(EVAL(THIS.ControlSource))
THIS.SelLength = 0
NODEFAULT
I didn't mention that you must do a DoDefault() at the start so focus will get to the editbox, but then you chage the selstart and sellength and issue a NODEFAULT to prevent the default setting from occuring after your settings. Essentially you are making the default code run BEFORE your code.
BTW, I also noticed that if you don't set the sellength the insertion point ends up inthe wrong place.