It appears as though there is a bug where a custom field name overwrites the contact name in the e-mail receipt that is sent to the contact. In the custom profile that is part of my membership signup page, the last field is titled "Interests". In CiviCRM itself, the data is saved properly, but in the e-mail receipt that is sent to the contact upon registration, the name of the contact is replaced by "Interests" in the Billing Address section:
Billing Name and Address
555 SW 5Th
Portland, OR 97233
I remember seeing another issue in the forums somewhere where the contacts first and last name was overwritten by the billing name, so I'm wondering if this is related.