Running either of those codecs at high bitrates would likely tax servers more than Blizzard is comfortable with.
I highly doubt any of the voice chat will run on existing servers. They will probably just handle the negotiation through their text chat backend, and every client open a new connection to a voice chat server. Whether blizzard is installing new servers (or allocating unused servers) in their existing clusters, creating voice only clusters, or outsourcing I can't say. But I am fairly certain no voice data will end up on any server that is running game scripts.
I tend to assume Blizzard's implementation will look more like a carrier grade VoIP reseller type solution (hardware based encoder/decoders on their side) rather than the various Ventrilo licensees who use pure software platform. At Blizzard's scale it makes more sense to use highend vertical hardware than tons of commodity servers*. But I may just be hoping for too much ;-)
*Strictly speaking it may make sense at the scale of some of the larger Vent OEMs, but Vent does not offer them such an option. Blizz can probably find carrier grade hardware that supports any codec commonly used for SIP or IAX2.