As I have been observing the Selenium 2 project very closely, I noticed that they are not following the "bing bang" integration approach, i.e. the features of Selenium 2 (which have its origin into Webdriver) started to creep into the Selenium RC server over the last one year.
Now Selenium 2 is officially out there, still not very stable and also the lack of documentation is a challenge. However whatever I got to test using the Selenium 2 has been very encouraging, gives you a lot of control and the "same origin" problem is completely avoided.
Now the big question, Selenium 1 or 2?
There in no Selenium IDE or GRID for Selenium 2 yet and that could be the deciding factor for many.
If you are hands-on programmer (esp. java) then you can directly start with Selenium 2. Otherwise, I will recommend that you start with IDE, record and run the formatted code in RC (Selenium 1) and then slowly migrate to Selenium 2.
Another deciding factor, since Selenium 2, even though released officially, it is still a beta release and it might be risky affair to run your client's test scripts on a beta release. So, definitely practice and learn Selenium 2, however wait up for it's official "full-blown" release at a later date.
Also if you need to use GRID then you need to stick to Selenium 1... at least for the time being.