Online membership renewal with price set fails to update membership record for anonymous user

Initial details on this bug on the forum at:
http://forum.civicrm.org/index.php/topic,19191.msg112575.html#msg112575

I recreated it on the 4.2 drupal demo as follows (including details and screenshot in case demo is reset):
1. Create price set with 2 fields:

• Membership field - radio button, two options -> General and Student memberships
• Additional Contribution field: text, price = 1, not required

2. Assign price set to sample contribution page 2
3. Set up Dummy processor and select that for sample contribution page 2
4. Create a contact - first, last, email
5. Logout and go to online membership page (http://drupal.demo.civicrm.org/civicrm/contribute/transact?reset=1&id=2)
6. Fill out form, use email from contact created in step 4. Include a $4 additional contribution.
7. Log back in and edit the newly created membership setting Join and Start dates to June 25,2010. Save. This will change membership status to Expired.
8. Logout and go back to online membership page. Complete the form again. Include a $6 additional contribution.
9. Login and check the contact's record. You will see both contributions completed w/ expected amounts. However the membership record is still expired / end date has NOT been updated.

I repeated this as a logged in user - exact same steps. This time it worked properly (membership was updated on renewal). Did a quick check in the affected Membership BAO and didn't see any clues as to why anonymous flow would fail ???