If you want to copy your characters over to the 3.0.8 PTR I would do it soon, as in the next few minutes. The copy is currently up and working via the account management page at worldofwarcraft.com (go there yourself and navigate to account management as to ensure there's no trickery involved in links, etc...)
The PTR client is not available to be downloaded yet, but we'll let you know when we hear that it is. Expect that to be anytime, probably before the end of the business day on the west coast.
There's a good bit of changes to check out over at the PTR 3.0.8 patch notes. Be sure you do so and stay tuned to WoW Insider for more analysis of specific class changes.