Why is voice-to-text input not working when disconnected from the Internet? As you know, on Nexus One you can use voice input to enter text by speaking.

Unfortunately, if you are disconnected from the Internet (even temporarily), this feature is unavailable.

Why?

On my Windows laptop I can do this without being connected to the Internet.

Can you explain?
The Android implementation of Talk-To-Text sends your voice to Google's servers, parses your speech into text, then sends the text string back to the handset.  Without access to the internet, it's unable to transmit your voice data to their servers.