Why not just export it as a conventional date, and then format it to display the way you want?
I'm not the end user so I have to keep him in mind, but also, when you export / save as a CSV after viewing, it recognizes it and changes it back to 8/20/12.
You could use:
Left ( MonthName ( Date Field) ; 3 ) & " "
& Day ( Date Field) & "," & " "
& Year ( Date Field )
...but I think you are being fooled by the cleverness of Excel. Is your calculation field set to return the result as Date or Text? I would still suggest you export it as a true date. Then downstream users can add, subtract, display, week number, month, year, quarter, and use all the other date functions of Excel from it. If it is just text, they are stuffed. If Excel's import is converting Aug 20, 2012 back into a date then there is no advantage in having that wrestle with Fm and XL. (And 'respect' to Excel.)
Thank you Sorbsbuster! I understand what you are saying. This Export is being re-imported into a text field to Salesforce, and they want it displayed as Aug 20 2012...but I'm with you on the other points. I have the calc result as text. Thanks again!