Currently when leads are merged all fields values are taken from the winning record, which is always the newest record or contact I think. This causes fields like lead source and SFDC created date to be overwritten with the newest record's data.
There should be an option to choose for each field, at minimum source & date fields, whether the winning or losing record values are kept.
I'd be happy with a default that kept lead source and create dates for the oldest record. But I'd really like to see the option to choose for every field.
Related discussion:
http://community.marketo.com/MarketoDiscussionDetail?id=90650000000PesYAAS