This would explain why I have leads with activities typically associated with a known lead (e.g. fills out form, Sent Email, etc.) that have dates before the Created date. I've noticed a similar phenomenon when converting / merging a Lead into a newer Contact record in SFDC. To preserve the original creation date, I created a "Lead Created Date" custom field, so I can detect when the Created date is incorrect due to undesired behavior during merges and converts.
Jack, please create an Idea for this as its pretty important.