Something I noticed when spec'ing out my priest this morning -- I was unable to spend points in any other tree except the one I picked. So here's what I experienced after this most recent build:
After selecting your primary tree, you then go to the following screen:

A few points to note -- a) Once you pick your primary tree, the icon for your tree automatically gets selected (as you can see the guardian spirit icon in my talent tab) and b) You are forced to select talents in the tree with which you've selected. This means once you "Choose your Specialization" you must then go back to the trainer to retrain a new tree regardless if you've spent any points yet.
Notice in the above picture I've spent 30 points, still needing one point to spend to pick up my 31 point talent -- Guardian Spirit. More importantly, you'll notice the talents of the other trees are
grayed out. This means, you won't be able to spend points in other trees, until you've spent 31 points in the tree you selected to specialize in. This is interesting and disturbing as this really forces us to spend 31 points in the tree of our choosing and prevents us from creating a highly hybrid tree. Keep in mind this is early beta, but it's interesting these "talent/tree locks" are in place in game.
Once I spent my 31st point to pick up Guardian Spirit, then the talents in the other 2 trees become available for selecting:

Finally, we have our fully spent spec:

I just thought I'd share with you how the talent tree interface currently exists. I'm sure MMO-Champ will cover this in another of it's Cataclysm Interface Changes.
Additionally, as a side note, I think it's bullshit as a priest that we are forced to pick up one of the shittiest talents (Lightwell) just to get one of the cool new talents for the Holy tree (Chakra)