We had a similar problem while performing merge replication between SQL Server 2005 and SQL Server Mobile, We only have few handhelds in the field but receive this error after every couple of weeks or so. There is not much information on it.
It is a client side error, ie no error shows up in the server logs. What we do to go around this error is to Reinitialize All Subscriptions, selecting the "Use a new snapshot" option with "Generate now" checked. Once the snapshot is done the device replicates again and is just fine.
Don't forget to monitor Snapshot Agent Status.
Please post here if you find a solution.