The lookup transform has a new property in SQL 2008 which controls how to handle rows with no matches – NoMatchBehavior. It has two values - “Treat rows with no matching entries as errors” and “Send rows with no matching entries to the no match output”.

From the BOL entry:

When the property is set to Treat rows with no matching entries as errors, the rows without matching entries are treated as errors. You can specify what should happen when this type of error occurs by using the Error Output page of the Lookup Transformation Editor dialog box.

When the property is set to Send rows with no matching entries to the no match output, the rows are not treaded as errors.

In the properties window, this shows up as a drop down list. However, it looks like we forgot to document the actual enumeration values you would use if you are setting the property programmatically.

The enum looks like this:

public enum NoMatchPropertyEnum : int
{
    TreatAsError = 0,
    SendToNoMatchOutput = 1
}

This info should appear in the BOL docs the next time they are updated.