Yes, it sure is an impressive number.There were 7972 DDI members also registered on the WotC forums on 2009-09-06. To get to 62396 today would require roughly an 8.6% increase per month, but that's slightly misleading, since the number doubled to 14392 between 2009-09-06 and 2009-10-06.
I'd estimate the average monthly growth rate at about 6-7% per month over the last two years, which does seem to be reasonably impressive.
It's rather fascinating to see this numbers through the years, even though it's hard to make a direct comparisons between magazine Dragon and DDI. However, I must say that one thing has remained constant for me. Back then, I was the only one in my gaming group with a subscription to Dragon magazine and, nowadays, I'm the only one with a DDI subscription...