You guys crack me up. My wife would likely give you a different answer.
I thought the syntax was what Bob indicated, but I had never used it. I just tested this rule and it works:
If Customer.Nbr>=1000 Then
Customer.Nbr_String=AS_STRING(Customer.Nbr,'#,##0.00')
ELSE Customer.Nbr_String=AS_STRING(Customer.Nbr,'#,##0')
I'll ask Vladimir to add some of this formatting info to the User Guide documentation of AS_STRING.