If I were playtesting, I'd want to take a shot at converting the game I'm in now (although briefly on hiatus), which would probably mean starting at 18th level. But how can I possibly know what a gaming group where I DM only rarely (and until starting a SWSE game a few weeks ago it had been 'never' rather than rarely) will be doing next May?
The way to bet -- assuming most of us like 4e and want to switch -- is that we'll finish whatever we're doing and start over at 1st level. But we might be in the middle of a campaign we like, and so will convert it in-place. And if I'm running things, I usually like to start at at least 2nd level because some concepts are inherently multi-class and I want to allow them from the start.