I develop code using .net, and sometimes using ruby. My friend wants me to develop a small time keeping application for use on a mac. Basically a windows form application. I want to develop on my pc (it has all my tools) - I know the writing/deployment process for windows. Is it different if I want to make it work on a mac? Does the CLR work with OS X? Do I need to install something special on my buddy's laptop?
You'll want to look into the Mono project -- this is a CLR compatibility layer for multiple platforms (including Windows). Be aware that Mono is not a fully compatible system, and you'll need to check that your application is Mono ready. This can be done by using the MoMA tool (also provided by Mono).
I don't know what would be harder, getting Mono setup and running on all the computers you need the program, or learning Cocoa / Objective-C for true native mac development.
If you end up wanting to do some serious, native Apple Programming, you should learn Objective-C and the Cocoa API. You could start here: http://www.cocoadevcentral.com/d/learn_cocoa/
Mac users are very particular and will notice if an application looks, or acts differently then what they expect, so if you plan on marketing said software, it could be a headache.
Is a web application an option?
Its platform independent and you could utilize your .net experience.
Mono is certainly a viable option. Since you also develop in ruby, you may want to consider the Ruby-Cocoa bridge. This is supported by Apple and will allow you to develop in ruby, but still make calls to the Cocoa framework to take care of more system/GUI-specific tasks.