I'm a dev in the Windows world.  I'm considering hopping to Mac. 
Cocoa sounds particularly good, but I'm not sure if it makes sense to
go that direction, because it's specific to Mac OS X.

Ballpark's fine on all these numbers:

What is the total number of active Mac users?  

What are the installed versions and percentages of installed base in Mac land?

OS 9?
OS 10.1?
OS 10.2?
OS 10.3?
OS 10.4?

This is fairly simple infomation which is widely available.  I am v.
disappoined in your answer.

chandlerjw-ga:

"
This is fairly simple infomation which is widely available.  I am v.
disappoined in your answer.
"

I think you must be confused.  This is a question, for which I am
interested in an answer.  There is not yet any answer to be
disappointed in.

If you meant that you are disappointed in my asking the question, um,
I'm paying for it, and what business is it of yours?

I would forget about developing anything for OS 9. It has gone by the
wayside and has been replaced by OSX. I am not a developer myself. The
mac market hes been on the upswing for the last few years. The 10. or
.2, .3 etc are pretty similar. The latter revisions are adding bug
fixes and features. From what I understand many Unix and Linux users
are adopting the Apple laptops.
 I would say go for it, especially if  it looks like soemthing you would enjoy.

Boneal

Here are the info: hope this will help u
---What is the total number of active Mac users? 
http://www.apple.com/pr/library/2004/jan/06macosx.html

Great link arvind97, and I am willing to bet that the installed base
has grown substantially since that time. Also, when 10.4 "tiger" is
released later this year, I am willing to bet that it will entice a
lot of people to switch. This is all speculation of course, but I
myself use 10.3 and am probably going to upgrade to 10.4 when it comes
out.