You are using the wrong expression for the format. Given a date of 2/3/2009, your expression

=Format (<datetime field from dataset>, "dd-MMM-yyyy").

evaluates to "3-Feb-2009" which is then used to format the date, which results in 03-2eb-2009.

Simply enter dd-MMM-yyyy in the format textbox or use expression ="dd-MMM-yyyy" will fix the problem.

Peter