Some publishers were able to block more than 200. In fact, I was able to block up to 500 since February 2009. However, now all publishers should be able to block up to 500.
Check your account. We have more than doubled your number of filters (to 500).
Please don't go crazy with this. The reason we're not announcing it on the blog is that we don't want to overload our system with everyone filling their filters to capacity at once.
Huge thanks go to the awesome engineering team that implemented this for us (I'm an AdSense publisher, too, after all).
Please feel free to leave love letters to our engineers below.
Most publishers are really happy about this increase.
Forum discussion at WebmasterWorld.