After upgrading a SharePoint farm to Service Pack 2 you could find out that sometimes your Site Collection is locked and no administrative tasks can be done. If you have scheduled backups using the stsadm, you will find out that this lock is active only when your back up jobs are runnig.
This happens because the stsadm command backup (stsadm -o backup) now locks by default your site collection. As you can see this option is implemented by default:

stsadm -o backup
 -url <URL name>
 -filename <file name>
 [-nositelock]
 [-overwrite]

http://technet.microsoft.com/en-us/library/cc263441.aspx

for more infos about the nositelock option, visit http://technet.microsoft.com/en-us/library/cc262811.aspx

Hope it helps.