I don't see any obvious technical limitations. The stored procedure might be more difficult to follow than an SSIS package for complex ETL operations - but that isn't going to be true for every scenario. I have also found that packages (SSIS and DTS) are more readily recognized as "jobs" - the stored procedures that are executed by scheduled jobs are often overlooked by developers because they can't see the scheduled jobs.
That said, I have seen ETL performed by stored procedures and DTS/SSIS packages alike and as long as the stored procedure isn't a large mess of tangled code it seems appropriate. I haven't seen one method perform better or more reliably than another (but then I haven't seen stored procedures doing complex ETL).
