Apple's third-generation iPad, officially available as of today, doesn't include all of the features we have come to love about Siri, but it does include Siri Dictation.
Siri Dictation is a voice-to-text solution, allowing you to speak what you want entered into a text field, and Siri will automagically type it for you.
You'll be given the option to enable dictation during the initial device setup. However, if you change your mind, you can disable (or enable) dictation by visiting Settings > General > Keyboard > Dictation.
With dictation enabled, you will see an extra key, displaying a mic, present on the keyboard. Tap on the mic to launch dictation. Once you are done speaking, tap on the mic again. You will see three blinking dots for a few seconds, then your text will be placed into the text field.