If they handle it like Entrenchment, the beta will be available whenever (1) you preorder Diplomacy or (2) when the beta first becomes available. Obviously, it'd be whichever came second. Basically, you should not have to preorder by a specific date to have access to the beta.
As a side note: you may have to change a setting in Impulse to allow you to see beta releases, but there will likely be half a dozen threads on the matter.
Edit: Stupid emoticons, messing up my labeling scheme...