Patch 4.0.1 came out on the PTR yesterday, and after hours (and hours, and more hours) of downloading it I was finally able to log on. Note: Don't try to download 16gb onto 3 computers at once. It doesn't work well.
I was happy to see that my 3 mains (druid, shaman, and paladin) had copied over successfully.
It's not beta (sadly) but it's still pretty cool and worth checking out.
The build on the PTR as of this moment includes the talent tree overhauls, class changes, the brand new stat system (no more defense rating, no more spellpower on most stuff, etc), spell changes (no more spell ranks!), new glyph system and changes (not fully implemented) and many of the new UI changes.
I've gone into each of my toons and spec'd them for healing, although I have yet to try an instance on my paladin. Druid and shaman healing is a lot of fun - I'm having more fun with it than I am on live servers, currently. The new spells and talents are very refreshing for someone who has healed on a shaman for nearly 2 years straight. I've druid healed since the release of Ulduar. I have yet to try out enhancement/elemental or the feral tree. I did spec for balance on my druid but at the moment Insect Swarm seems to be bugged and I wasn't able to learn it from my trainer, so I haven't really done much with balance yet.
At any rate, from looking at the PTR I think Cataclysm is going to be freaking awesome :3 And I can't wait to stand in line for hours to get my greedy little hands on a copy.
P.S. Fun fact - While you cannot yet fly in vanilla Azeroth, you can mount up on your flying mounts. Watching my proto drake "jump" makes me smile.