Someone on Reddit is a freaking genius. One ingenious user, using a throwaway account, has created a GLaDOS voice pack for the Garmin GPS.  The file itself is a zip packed with .wav files that are compatible with both Garmin Nuvi and Garmin Mobile XT units, and is totally free.

There's more information on compatible units over at the download page. I don't know about you, but I'm not so sure I want to find out what she'll say if I miss a turn and need to make a U-y...