As much as I love Yoda's answer, I really don't think that should work.
As I see it, minimum length comments prevent very low quality comments to appear in the sites. If somebody takes the time to generate the comment to hide text and circumvent the limit, then most probably the comment has some thought also and it will not be of such low quality to make it unfit to SE sites, regardless of its length.
To circumvent the limit anyway, it is always easier to write (10 characters) as it's customary in many fora (I have used it in SO also once). Of course, there can be yet another script to strip it, but "characters" is a word that could be very well be used for real content also.
So, in conclusion, I do not see any improvement in implementing this feature, and if it is implemented, I see too many ways to circumvent it.
Juan made a very good point asking how would you prevent it, and Yoda made his point very clear also. How many comments do you think you'll find like:
Can you post full code? <!-- Very clever way to avoid having to type a few more characters in my comment asking for da codez -->