With a HD-Tivo, there are no delays!
I just got mine set up and with my std-def tivo using the digital output anything would cause the signal to be lost (I think the old device turned off the output when fast forwarding and other stuff) the new one the audio continues without the usual signal lock pause...
So, if that was keeping anyone from getting a HD-Tivo run out and get one now! and it's a REALLY cool device