It's a pretty nice idea, indeed
It would be even nicer if you could administer the milestones these are sent on from the AdminCP, instead of them being hardcoded.
Similarly some admins may want to have a PM sent with the congrats, instead of an email. Or if the user chose to not receive email from administrators, in which case it could also revert to a PM.
Additionally, the actual text emailed/pm'd and the subject can be set from the AdminCP as well. Username and Post Count could be variables in this text so it can be personalised to a degree.
Would any of these extras be considered overkill, because I'm contemplating of writing this hack