We actually did some research on this some years ago as a school project ("typing speed on top number keys vs. on keypad"). We set up in a busy intersection and asked people to type 20 6-digit numbers into a simple application. Gave them a cinnamon roll for their efforts.
We measured speed per digit, speed per number and speed per 20 numbers and number of typing mistakes. Also, we asked the subjects about whether or not they used a PC regularly and whether they were lefthanded.
It turns out that for left-handed people, the numberpad is consistently slower than the top number keys by about 25% for the people who are using a computer regularly.
For righthanded, the difference was more marked in favor of the numberpad - it was something like 40% faster for the people using a pc regularly.
If a user didn't use a pc regularly, regardless of whether they preferred left or right hand, the numberpad was slightly in front, about 10% AFAIK.
As far as I can remember, the conclusion was that since most numbers start with digits in the range 1-4 [1] and they're close to a lefthanded users' strongest hand, one uses the keypad significantly less than righthanded users who can't reach 1-4 on the top row with their right hand and who therefore find it easier to just move the right hand to the keypad every time.
Inexperienced PC users found it easier with the numbers all in reach.
[1] http://en.wikipedia.org/wiki/Benford's_law
edit:clarification