Prefill Twitter Whats Happening Field

I need a simple yet compelling task: the ability to prefill the "What's Happening" field when posting from my website to Twitter (similarly for Facebook, and MySpace etc.) The text to pre-fill with, will be determined dynamically for each page on my site, and passed in to your Javascript function dynamically e.g. prefill Twitter("send this text to twitter") Is it possible to have something like <span style="background-color:#ffff00;")send this text to twitter</span>. If it is possible, then you need to accomplish putting a background color on the tweeted text. First person to do this for $30 wins the job... --