It's a really excellent question - I'd heard originally - I believe it was from others who'd spoken with Lotus USA but could not swear to it - that the engine choice for the US version Elise would be announced in or around July, and the rest of the specifications would be released subsequently, and pretty much all would be known by October/November time frame.
We're all eagerly awaiting official word from Lotus on these details, but so far it's pretty much rumours. We all just need to try to be patient, and it's not easy !