Does anyone know if any ezines explicitly offer split-run testing?

That's where half of the ezines which go out have one ad, while the other half have an alternative ad.... This way, you can see which ad gets more response, and is therefore more effective.

I just realized, it's relatively easy for me to do split-run testing with our ezines here (the SeedZine and the Entrepreneur's Hotsheet), and I figure it probably ought to be easy for other ezines to offer this too.... Has anyone heard of any ezines offering this?

Thanks. :)

- Dien Rice