We have an application that uses the DTS infrastructure to import data from a variety of ODBC accessible data sources into a sqlServer warehouse repository.
Excel spreadsheets have always been a pain in the datatype and column name category when the same worksheet is used to import data over and over. I find the datatypes (and type control) in Excel is simply too fluid to be use for anything more than "one time" imports and even then, you're still likely to run into data type problems.
I always try to find a way to get Worksheets into something more "stable" before attempting any sort of data manipulation on them.