Excellent question.
I wish I would have thought of this issue in advance of the migration, to raise it earlier.
Today, I had a top user of the SE 1.0 site, who did not commit to the proposal at area51, get in touch with me to express concern about why they couldn't log in to the site.
So, in hindsight, I think that registered users of an SE 1.0 site being migrated should be made able to access the SE 2.0 version of the site during private beta, as if they had committed to the area51 proposal ... i.e. grandfather all SE 1.0-registered users into the SE 2.0 private beta. After all, many such users were committed to the site already, if not committed-in-fact at area51.
Even though I had a "support our proposal" link pasted at http://basicallymoney.com, I suspect users (even regulars) that didn't grok Stack Overflow didn't bother to commit at area51.
As for unregistered / new users, I agree with Robert's answer, to keep it somewhat limited for testing.