Whilst I understand that Mint have consciously chosen not to have a release schedule, I feel that you are missing out on a major promotion tool.
My main source of Linux news is Distrowatch and their weekly newsletter. I like to browse the "upcoming releases": http://distrowatch.com/weekly.php?issue=20110418#upcoming
. My favorite distro Mint is never listed, even when new releases are coming up. A missed opportunity.
Also consider that planned launches can create excitement and press.
Then there is also the fact that not having a release schedule other than "about a month after Ubuntu releases" looks unprofessional.
I think this would be a simple step that would be well within the growing Mint communities abilities. At the very least, someone should email the Distrowatch maintainer and give him some kind of provisional release date at least so that Mint appears on that list.