You can use an auto-enter calculation in Field options such as this:
Self < Get ( CurrentDate )
If you are using an older version of FileMaker that doesn't support "Self", substitute the name of the field being validated for self.
"On some date fields I'd like to block users from entering dates that are in the future. How can I do this please? They should be able to only enter today's date or dates in the future."
I take it you want users to be able to enter today's date or dates in the past. If this is the case you can use Phil's formula as Validation by calculation. I won't work as an Auto-enter. . . unless I'm missing something here.
Thank's Rick, I'm still a bit fuzzy headed courtesy of a head cold. I meant a validation calculation not auto-enter.