Excel doesn't always maintain variable properties as it goes back and forth between programs.
If the zipcode in Excel is being displayed with the preceding zero because the zipcode format has been applied to that cell/column, then the data is still considered numeric.
Also, even if the zipcode data column has been changed to text format, your data may not retain that property when being read into another program.
For ArcGIS, I always save my data tables in dbf format. You can save your excel file as dbf, but check to be sure the preceding zero was maintained. Also, you can always convert your Excel table into an ArcGIS table via ArcCatalog. This would be the preferred method to ensure compatibility between your table and ArcGIS processing.