I've changed that bit of the form into the separate fields, but where am I concatenating the values? I tried it in the destination page by writing
<CFSET dob = Form.birthday & Form.birthmonth & form.birthyear>
<cfinsert datasource="062105cs06sr" tablename="users" formfields="username,createDate,password,firstname,lastname,#dob#,town,postcode,bio">
But it's not working. As you might be able to tell I'm a bit of a novice at this..! The form worked fine before trying this.