I'm getting the same symptom as Mike, so I'm glad you've already got it figured out! I don't usually pay much attention to this part of NMU, since I have unlimited SMS and data on my plan and there's no "unlimited" option--I do kind of like to track my usage, but the length and color of the bar tends to be misleading fo unlimited plans. I've kluged it by using the arbitrary numbers 10GB data and 10,000 SMS messages...but it IS a kluge.
Any chance we could get an "unlimited" option in Settings in a forthcoming version, that would always render the bar in green? Not a big deal, since that would just make YOU pick an arbitrary number big enough that it would never fill up, but SMALL enough that you could see the bar making its way across the screen! But it would at least be more intuitive to the user than lying to the app and making up a number to put in that field!