The following link will walk you through moving the logs. Note that the database will be unavailable during this:
http://support.microsoft.com/kb/821915
Or you could take a full backup to flush the logs.
Tim Harrington | MVP: Exchange | MCITP: EMA 2007/2010, MCITP: Server 2008, MCTS: OCS | Blog: http://HowDoUC.blogspot.com | Twitter: @twharrington