Android's Voice Actions have been around for more than a year now, letting you send emails, navigate via GPS and making memos, but it still lacks features that Siri introduced on the iPhone 4S. Good news: Google has an upcoming update that makes Voice Actions respond to natural language requests, rather than you having to learn a bunch of pre-set commands. Even better news: They're calling this update Majel.
Android and Me speculates that the release could be in late 2011, but an early 2012 release in January or February might be realistic for the update. Other than that, there aren't a lot of details on functionality. This page shows what Voice Actions can do now.
It's too bad that Majel Barrett-Roddenberry (the voice of the computer from Star Trek) isn't around anymore, because having her record all the possible responses, or somehow synthesizing her voice to be the voice of Android's "computer" would be amazing.
Google's response to Siri is codenamed Majel, could be released by end of year [Android and Me via AndroidOS]