Safe to start Mirroring on Production Server? We have a 350 Gig DB on SQL Server 2008. We intend to backup and restore it then immediately start mirroring.

Is starting a mirror a dangerous operation to do on a production database ? Can I expect time-outs and do I need to do this in a maintenance window or is it a relatively safe operation to perform at any time ?

Thanks !
Assuming you have everything properly configured, starting mirroring simply means the log rows for transactions not already at the mirror will be sent to the mirror database; once that has occurred the only further overhead would be writing to the mirror database.  If the mirror is not very far behind the principal, then this step is generally very quick.

If you have mirroring configured in Synchronous (high safety) mode, that overhead might be enough to show delays in commits. Generally, transactions would be slightly slower since data must be written to both the primary log, and the mirror log before control is returned to the user. 

If you have a maintenance window, it would be preferable to start mirroring during that window.