The reason I have no sympathy for parking complainers is that I have a car in the city and I generally get one ticket every 5 years or so. If you bother to follow the law, once in a long while you'll get unjustly ticketed (and it's easy enough to write a letter and get it dropped) -- but it's overwhelmingly the case that most tickets are earned.
doomie, you're missing the point here. MD and VA residents who occasionally come in to the city are labeled as car registry offenders and need to go out of their way to prove their innocence when they have done nothing wrong. these tickets are NOT earned.
out of towners are allowed parking in the city ONCE every 180 days? that's asinine. per the wording of the ggw's OP, "Your vehicle has been observed for the second time within a 180 day period parking without DC tags." he now has to trek to a DC DMV with his mortgage papers/title/utility bill/first born for the "right" to not be ticketed for something he isn't guilty of. once he's done all this he'll be in the clear - for six months! then he'll need to do it all over again. that's an unfair burden on the innocent.
on any given day, i suspect there a lot more MD & VA day-trippers in the city than there are registration dodgers. in the city's blood-thirsty pursuits of the few, they're punishing the many.
that being said, i've had MD plates on my car for 2 years now, i go in to the city at least once a month (and often more than that), and i've never received such a warning/notice.