I'm writing comma delimited strings to .csv files in Excel via streamwriter. I'd like to split up the output on to multiple worksheets within the same workbook? Any thoughts?

if you are end-user, please just see http://office.microsoft.com/en-us/excel-help/import-or-export-text-txt-or-csv-files-HP010099725.aspx.
if you are doing this in C#/VB.NET, then  please see the below:
1 a similar discussion 
2,  another CSV and Excel article based on 3rd tools that may help you a lot.
Best regards