It will pre-cache the entire route before you set off to help with signal black spots, so you only need a connection for the initial routing planning stage. I'm not sure why people are that worried about live map downloading though, GPRS should be fine, even on the move. Basic map data is not huge and can be well compressed. I have done this with Nokia Maps a fair bit and it was fine. Pre-caching satellite and street view images is not realistic anyway, it would take huge amount of storage. But i agree, having the maps pre-loaded does give you an extra piece of mind.
Google maps uses Tele atlas mapping (owned by TomTom) AND NavTeq (owned by Nokia) so it will be interesting to see how well that works out if they use both for this. I dont know of any other sav nav that uses both.