While at Bronto software (http://bronto.com/), one of the features I worked on was just that. We called it Send Time Optimization, and from what I understand, it's not an unheard-of feature amongst email software providers.
Completely off topic — you worked at Bronto? I did a short (like 1 or 2 day) contract to write a first pass at their Ruby client gem a couple years back. Seemed like nice guys. I get shivers thinking about Ruby and SOAP though. Eesh.
Silverpop has it & also calls it send time optimization. Allows for optimizing the send over 24 hours or 7 days.