Saw this question had been here for a while so I'm just throwing some ideas out there...
I'm wondering if you could write some code to use the DTS API to call your packages, then write assertions about those packages in the unit test tool for whatever language you used. For example, you could write your code in C# and use NUnit.
Additionally, if your DTS packages are all just calling SQL stored procedures, you can unit test the procedures (which would be true unit testing) using T-SQL Unit.
If it's higher level (integration) tests you want, then you might also consider hooking FitNesse up to the DTS API.
Not sure if any of this helps, but I hope I've at least given you some ideas.