What does "mds: (Error) Volume: Could not find requested backup type:2 for volume " signify? In Console.App I can see various log messages.  I've diagnosed a Time Machine problem and searched for "backup".  In the resulting messages from backupd I also noticed the message

27/07/11 16.01.59,562 mds: (Error) Volume: Could not find requested backup type:2 for volume

showed up (which is most likely not Time Machine related but has the word backup in it).

Any suggestions on what this is and how I can fix it?
I have only seen that in relation to Time Machine timing out when preparing a network sparse disk image for backup.(and I've seen it on Snow Leopard and rarely on Lion - but lion is young)

mds is very tightly intertwined in the time machine process, so it could just be time machine asking mds to do some work. 

You'll have to disable time machine and watch for other network mounts failing - separate the two and you'll have narrowed it down to one bad actor.