EDIT: Just found a reference article from the SDTimes which referenced some of Evans Data 2009 report. The full 63 page report can be purchased from Evans Data and to clarify I have no link to them.
For 2009, Evans Data originally
estimated that there would be
approximately 15.2 million developers
worldwide. However, it has reduced
that estimated by about 600,000 in the
current report. In North America,
Evans Data projected in a previous
report that the developer population
would grow to 3.85 million in North
America during 2009. In the current
report, it changed that figure to 3.72
million based on current economic
conditions. Evans did not disclose
data on other regions.
This is the previous article I found with a referenced number of 14.5 million in 2007. Reference www.infoworld.com & Evans Data.
New research shows that more software
developer jobs will be created in Asia
than in North America by 2010.
The worldwide software developer
population is expected to grow to 19.5
million by 2010 from 14.5 million in
2007, but North America will account
for only 18 percent of those jobs in
2010, down from 23 percent today,
according to statistics from Evans
Data presented at a conference the
research firm is hosting Tuesday in
Redwood City, Calif.
While the North American share of the
developer work force will shrink, the
APAC (Asia-Pacific) share will grow to
close to 45 percent from 37 percent
today. The share of developers from
EMEA (Europe, the Middle East, and
Africa) will slip to 30 percent from
35 percent, and the share from Latin
America will remain flat at 6 percent.
It doesn't come near to Joel's numbers so he must have been referencing some other source. This second earlier article does provide detail to answer your question on distribution though.