Everyone can stream in 1080p in perfect quality. You just need the hardware and the bandwith to push the quality.
Mostly that means: at least 4mbit/s upload (stable) and a decent overclocked FX-8350 or i7 (sandy, or newer) CPU to handle the x264 load while encoding 1080p 30-60fps on the fly.
The other thing is being able to watch it.
1080p hits hard on the hardware while decoding it, so people with slow systems and/or mobile devices can get in trouble.
Second one (affects twitch.tv mostly) is the delivery of the videofeed to your viewers. You can send 1080p without problems, but not everybody would be able to watch it without buffering because ISPs cant get their crap together and transfer a live video without buffering to the people from all over the world.
Partnership on Twitch.TV just means that you a) have a transcoding option for people with shitty hardware/connection, so they can lower the quality. And it will be avaliable anytime, for non-partnered people this will become active after they hit a decent viewer number live. And b) you have to option to monetize your content via ads and subscription (4,99$, 50% of it is yours) and get some additional benefits like smileys, sub-icon whatever.