I don't blame Twitter, per se. But you've got to admit that the service loses a bit of social utility if it acts more like a documentation resource for the government than an anonymous communication service.
I mean, if you're going to make a stand, you'd figure doing it against a subpoena for what are essentially traffic violations would be a nice place to start.
The best part is, thanks to the USA PATRIOT Act, cops don't even need a judge or a warrant to get it anymore.
http://en.wikipedia.org/wiki/National_security_letter
From 2003 to 2006, the FBI averaged about 65,000 NSLs sent - PER YEAR.
Twitter has the username to ip+timestamp mapping, your ISP has the ip+timestamp to name+physical address mapping.