Move SQL Server 2008 database from machine A to B with no downtime
Database Mirroring (SQL Server 2005+) would allow as little as 20 seconds of downtime.  This is likely the fastest way of moving a database from machine A to machine B.

You can setup mirroring during the day with no downtime, then cutover at the time of your choosing.  As long as the clients are programmed to retry whatever operation they are doing at the time of cutover, there will be no data loss (if you do the client correctly).