What expression are you using? (Since you are on Mac, this may be a platform specific issue also.)
When I launch FMP 10 on windows XP and use either of these two auto-entered calculations:
name & GetAsText ( Date )
name & Date
Where name is a text field and Date is a date field, I don't get any leading zeroes unless I manually type them into the date field. I get identical results for Filemaker 11.
The expression for auto-entering the concatenated field is "production_name::PROD_SHNM& Engagement_End". "PROD_SHNM" is a five character text field. "Engagement_End" is a Date field. The date formatting for "Engagement_End" is working correctly, either by typing it in or via a drop-down calendar. When the concatenation happens, the leading zeroes vanish. I've also tried GetAsText(Engagement_End) with identical results. My System default is also MM/DD/YYYY. "Always use file's saved settings" is checked in File Options to allow for the solution to be accurate on a machine with different system defaults.
Found the problem: using a drop-down calendar doesn't work as I expected, even though the date LOOKS to be correct. That was just layout-level formatting. The date DOES have to be typed in.