This error is most commonly experienced by Lotus Traveler users.
A 409: conflict status when returned by the server typically means that synchronization was initiated, but the device connected and initiated a new synchronization before the last one finished. It is a common error and usually transient. If a high percentage of syncs end in 409 then you may want to troubleshoot further to improve performance. See the IBM Info Center topic link that has an example on "show stat command" that can look at the percentage of syncs that end in 409 (conflict) vs. 200 (successful).
Note: In some cases, the folder throwing the 409 errors may be having issues because it is not located at the proper level. Please make sure the Contacts, Calendar and Mailbox folders are all at the top tier of the folder system. In some cases, even the Drafts folder needs to be located at the top tier in order to avoid conflicts.