I want an operating system that can be used for both gaming and developing. Is Mac OS the right choice? I'm both a (newbie) developer, student and a gamer. Right now I have Ubuntu 11.10 and Windows 7 installed on my box. I usually run Ubuntu for my everyday chores, but when I want to play games or type a damn report, I have to switch over to Windows (Sorry, I'm still trying to get accustomed to OpenOffice :)) Since I am going to reinstall my OSes, I want to look at all my options. Seeing that some games I play - StarCraft II, LOL, Diablo III - already supports Mac OS, so I am thinking of giving it a try as well.

I'd like to know if developing the same technologies using Mac OS is anything different from using Ubuntu. The technologies I currently have my hands dirty with and are especially concerned about are: GNU C/C++, Boost, Cassandra (Thrift, LibEvent, Flex, Bison), MySQL, Java, Android, Python (Django) and PHP. (Especially Cassandra, because I need to compile it from source for a project.)
Most or all of the software that you've mentioned is available for OS X (except LoL, which you can run through Bootcamp under your Win7). OS X is a nice operating system with a lot of very well designed software, but the best way to see how it works is to actually use it for a while. I have used Linux and I believe that OS X is much more polished but some people might not care about this and would love to have the ability to tweak every single aspect of their OSs. 

All Blizzard games run perfectly on a Mac. I do Android (and iOS through Xcode) development using Eclipse and I have no problem using OS X for all my needs.