Now available on the US App Store: Sina Weibo 微博 download here: http://itun.es/igk6zp then follow me at http://weibo.com/2140336255 and see why 200M users like it.
Note: to create an account you'll need some rudimentary knowledge of Chinese (either from something like Google Translate or from a helpful friend). I assume they're working on an English version of the web site too of course.