<p>How long does it take for an experienced Windows programmer to learn writing simple software for Mac OS X? </p>

<p>Where does one start and what tools will be necessary?</p>

THIS
.  I came from a Win32 API background programming lately a lot in C# and I was able to pick up on Objective-C and Cocoa very quickly by working through the book.  You will be pleasantly surprised, the APIs are very rich and just wonderful.  Reflection is a first class citizen which makes things very easy.
Xcode is of course free and available on your Mac OS X installation disk.
Edit: it took me about 2 weeks to come up to speed and write simple applications.  I think this is because the language and API are easy to learn and use and
Aaron Hillegass
's
book
was a big help.