Should I take any considerations for my Rails 3.0/3.1, Ruby 1.9.2 development machine running OS X 10.6 Snow Leopard before upgrading to OS X 10.7 Lion?
Will there be anything that I need to reinstall? Finally should I backup my gem sets somehow?
OS X (including Lion) comes bundled with it's own version of ruby, which will probably overwrite your current version. (I'm not sure if Lion is using 1.8.7 or 1.9*) But the worst that could happen is you'll have to upgrade to 1.9.2 after upgrading to Lion.
This shouldn't be difficult, but you may also have to reinstall all your custom gems, etc. I would suggest using RVM to manage Ruby independent of the local version on your machine. It painlessly allows you to keep track of any version of ruby you want per application so you don't have to fool around with your machine's version and other applications that may depend on it.
IE You could have one application using 1.8.7 and another using 1.9.2 each with their own set of gems and no conflict between them or your machine which may have it's own version as well.
There's a screencast demonstrating RVM on Railscasts.