So, one of the pain points in SSIS is that the Lookup Component performs CaSE sensitive matching, as well as retaining any trailing spaces that may be in the data. So be forewarned that when using the lookup component, it may behoove you to use a derived column upstream to TRIM([SSISColumn]) your data to get rid of any leading/trailing whitespaces and to perform the same (if needed) on the SQL Server side via an SQL statement – ltrim(rtrim(SQLServerColumn)). Also, if you have differing CaSEs among the incoming data and the reference data, you may need to convert one or the other, or both to UPPER/lower case before performing the lookup.
If you’d at least like to see CaSE INsensitive searches, please visit my submission and lets see if we can get this feature added to SSIS in SQL Server 2008.
One workaround for you, should you want to use it, is to use the Fuzzy Lookup Component instead. This has its own problems, as I’ve made mention to in the above feature request submission.
Edit: Jamie Thomson submitted an identical request before me but I’ve made reference to that submission in mine. The idea was to get it in SSIS 2008. Never-the-less, Michael Entin claims this feature request has been dropped due to lack of time.