I think this would be a really helpful metric, since it's something that we don't currently have any insight into. For anyone who hasn't seen the SilverPop email benchmarks, it's a great breakdown by industry of email benchmarks. You can
download here.
Another helpful thing (now that we have mobile information) would be a clearer distinction in where an email was opened or clicked. I know we can create reports in analytics, but it would be nice if there was a breakdown of mobile vs desktop in the email send section. eMarketer's annual email benchmarks report just came out last week, and is showing that mobile optimization is, as we know, very important.
Here's a chart from that report.