Are abstracts confidential during the review process?
I checked the peer review policy of Nature and Science, I guess that should is the best we can get. The policy can be found here, here (both Nature) and here (Science).
A bunch of relevant quotes from the Nature policies:

As a condition of agreeing to assess the manuscript, all reviewers undertake to keep submitted manuscripts and associated data confidential...
Nature journals keep confidential all details about a submitted manuscript and do not comment to any outside organization ...
Referees of manuscripts submitted to Nature journals undertake in advance to maintain confidentiality of manuscripts and any associated supplementary data.

And some from the Science review policy:

Reviewers are contacted before being sent a paper and are asked to return comments within 1 to 2 weeks for most papers.
The submitted manuscript is a privileged communication and must be treated as a confidential document.

I would read it conservatively. That is, consider an abstract to be a part of the manuscript, hence the same rules governing general manuscript should apply. Apart from that, both journals state that in advance to being sent the manuscript, the (I guess still potential) reviewers agree to keep the matter confident.
I think an answer may also lie in the exact process handling review. First, a potential reviewer is invited and sees a title and possibly paper's authors. Then he/she has to log-in into a review system, but I guess, somewhere in the process accepts general terms and conditions which established the contract. And only then the reviewer sees the manuscript. I am not sure if all journals do it this way, but those I reviewed for did so. Well, besides some special issues handled outside the journal submission/review system, where the case would be unclear.

I just checked the reviewer invitations I received from journals in my field and I found that the only disclosed thing in the invitation e-mails was the submission's title. I must accept/decline to review and only afterwards I saw the manuscript, or its abstract.