Lots of bandwidth, lots of cost.
YouTube has really the only genuinely successful model at this that's really out there - and it just so haapppppens that they own their own carrier network:
http://www.fixedorbit.com/AS/36/AS36561.htm
If you were as much as a geek as me - you might also notice that they're
only peering with the very cheapest carrier networks out there by that information (Cogent, Hurricane Electric, Limelight).
Google's video deal has picked up a little steam on YouTube recently as well, but you might remember that them buying up massive amounts of dark fiber not long before that launched (try 'Googling that' for some irony). From there, sites get by by just running tons of ads, in and around their videos, but there's no doubt the bandwidth costs are not at all easy to support financially no matter who you are.