I ran the 720p on Uncapped cos it's rate limited to 4Mbps (and I've also asked the Dev team to shape it permanently for me for testing purposes). I tested 1080p on capped, cos there is no rate limit and my line goes high than 4Mbps.
I let the videos play immediately (did not pause to allow to download) and most of them loaded the entire file well before the stream caught up. On 4Mbps Uncapped, didn't load ahead as much, but the little grey line was always well ahead of the play indicator, so no buffering really.
I was mainly watching HD trailers for Transformers, The Equaliser, Walk Among Tombstones, etc. I think I did a YouTube search for HD trailers.
Just a note there though - I pretty much turned everything off on my PC, opened a fresh browser and also disabled WiFi on my router. Only had YouTube and Bitmeter OS running on the PC. I'll test again tonight with more applications running (I'm a tab whore when it comes to browsing) and see if that makes a big difference.