It will not 'replace' the PPO, as it will only be available on certain, pre-chosen products. If an edition size is set prior to the pre-order beginning, I'm sure we will still run a PPO.
And yes, this has been introduced for items that you can only order from Sideshow - not regular edition items that you can buy in a store or from a distributor.
Also, Nerd, to address your question - it actually means LESS hurry up, buy now before it's too late, as you won't be competing against other collectors for a pre-set limited number. Basically, it's a timed exclusive - if you buy within ______ days, you will have your pre-order reserved, and you won't have to worry about re-adjusting your schedule to make sure you can be around at 10am on Fridays... don't worry, all of the required information will be announced with each item's preview and it's actually MUCH simpler than PPO