You can click this link to test access to a particular web site from within China. If you get an error message on a valid site name, then it is pretty safe to assume the site is being blocked (presumably by the Chinese government).
Here is the result for twitter.com: couldn't connect to host
Here is the result for markschulz.com: OK
Evidently, Hu Jintao has deemed the Stream Pool safe for the Chinese people (for now).
* Thanks to Dave Winer for the link