Likely you have bad data that cannot convert. Dates should never be stored in varchar becasue it will allow dates such as ASAP or 02/30/2009. Use the isdate() function on your data to find the records which can't convert.
OK I tested with known good data and still got the message. You need to convert to a different format becasue it does not know if 12302009 is mmddyyyy or ddmmyyyy. The format of yyyymmdd is not ambiguous and SQL Server will convert it correctly
I got this to work:
cast( right(@date,4) + left(@date,4) as datetime)
You will still get an error message though if you have any that are in a non-standard format like '112009' or some text value or a true out of range date.