The German company CLT Sprachtechnologie GmbH offers Dialog OS, a product for controlling technical devices by voice. The interesting thing here for the NXT-Community is the in-built support for LEGO MINDSTORMS NXT. Next to voice recognition, i.e. control of the NXT by words spoken by the user, there's also speech output on the NXT "in supreme quality" (as CLT states) .
Sounds interesting - have a look.