Hello
I rechecked this issue again and was unable to reproduce the bug on my local(CiviCRM v4.2.2). As a part of replication I followed these steps:
1. Created a profile of type membership and added some membership fields like member since, source along with the membership custom fields.
2. Included this profile in the Membership contribution page with paypal as the payment processors.
3. Did contribution as an authenticated and anonymous user.
can you please provide the steps you followed or some more information
checked in r42277.