Well, actually, I thought, "Seth, this is nice and all, but can you provide me either first-hand experimental results or published results to convince me that seven is the magic number?" That's close enough though.
If you want to look up the original research, and get some idea of when the conclusions are relevant and when people are just quoting statistics without really knowing what they're talking about, you could start here:
http://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_...
Positing the existence of the decimal system because humans evolved with ten fingers is something I can get behind. The seven-digit phone number, on the other hand, I'd probably attribute to other causes.
The seven-digit phone number in the NANP most definitely did not come about because of this; in fact, most quoted phone numbers from the early days of telephones in the US had five digits. You can see parts of the numbering plan reminiscent of this even today.
Case in point, I grew up in a St. Louis suburb called O'Fallon, where the predominant telephone branch exchange was 632. The first two digits correspond to OF as found on the keypad or rotary dial. From there, the extension was five digits. Belleville is very much the same way with the 23x exchanges used there.
If you were calling to someplace out of town, you'd name the branch exchange and then enumerate the extension. For example, to call the Belleville municipal police department from O'Fallon, you'd tell the operator that you're trying to dial "Belleville 4-1218."