31/8/2011
Also you can't use dd/mm/yyyy format. The standard functions expect values to be in U.S. date format ie mm/dd/yyyy. If your CF server is running under a locale where the standard is dd/mm/yyyy (like in England) then use the LS functions to parse the value first.
http://livedocs.adobe.com/coldfusion/8/htmldocs/help.html?content=Tags_p-q_18.html
<cfqueryparam value="#LSParseDateTime("31/8/2011")#" ....>
If your locale IS English/US, then you'll have to split it into month, day and year manually. Then use createDate() to construct the date.
<cfqueryparam value="#createDate(year, month, day)#" ....>