Custom Field values from Profile in Contribution Page not saved in new Membership

When using a profile with custom fields of membership in a contribution page these custom field values are not set in the newly created membership.

Steps to reproduce

1. Create a Custom Field for Memberships
2. Create a Profile that includes that custom Membership field
3. Create a Contribution Page for Membership signup and include that Profile in the Contribution Page
4. Use the Contribution Page to create a new Membership for a Contact

Expected Result

Membership is created and has the value of its custom field set accordingly.

Actual Result

Membership is created but the custom field is empty.
The Contribution Page's confirmation page does show the custom value however.

confirmed by another user, also see
https://civicrm.stackexchange.com/questions/10487/set-custom-field-during-membership-signup-through-a-contribution-page/10494#10494

If someone can give me a hint where to look, I am also willing to dig into the code myself.