360 Mbps now, probably 100 Mbps with newer compression
I think that with new developments in compression, (and chips decompressing it live in your tuner unit) a Ultra HD stream can probably be brought down to about 100 Mbps without any noticeable loss in quality.
That said, if eventually we develop a good standard with more-or-less 100 Mbps as the bitrate, most cable carriers will be able to carry at least 10 UltraHD stations with some changes to their current network, and probably a lot more if they get in to upgrades to 1.4, 1.6, Ghz and beyond. FIOS will also be able to deliver this over their network. DirecTV has already said that on their next satellite launches (2014, 2015 and 2016 will see new launches) they will be "Ultra HD ready", so they can probably carry a pretty good number of Ultra HD channels, and I am sure Dish Network will also be able to.
Who will be lagging behind? AT&T U-Verse. Without updates to their infrastructure, they will not be able to carry even a SINGLE Ultra HD station and deliver it to their customers.
"I reject your reality and substitute my own!"