Gone are the days of inpage; now writing Urdu universally in unicode is rather easy on both mac and PC (and Linux). Yesterday I was asked by a friend regarding how to tweet in Urdu. Here is a short 3 step guide without reinventing the wheel.
1. If you are not familiar with Urdu phonetic keyboard, the quickest way is to use Google Transliterate http://www.google.com/transliterate/Urdu
2. Urdu keyboard is quite easy to learn. If you want native support of Urdu in your operating system, follow the steps according to your OS.
Since I use both MAC and PC for native urdu support, I have tried and tested both the mechanisms and they work just fine.
3. If you are using transliteration, you would copy and paste the Urdu text into your favorite twitter / blogging client. Not all clients support unicode; tweetdeck didn't use to but twitter.com does support Unicode.
Happy tweeting / blogging.