Well, almost both!! But i also heard that last year, NSERC had some problems/delays, and that usually they reply with letters (rejections or acceptances) after mid-March. So, let's hope there wouldn't be any problems this year as well.
You mean they applied for NSERC, but live in US? and already got the letters?! :S that's interesting!
The website does say "by March 31", But this "by" may be very ambiguous! I was checking last year's forum, and it seems that it is mainly based on your location (due to the mailing delay).
I am not sure if it will be the same this year, but some people got their letters mid-April!!