Can we move mailboxes from 2003 to Exchange 2010 using this method? I realise the content won't move with it - but in a disaster scenario would that work? I assume we would need to use the "-ApplyMandatoryProperties" switch as otherwise they'll still show up as legacy mailboxes...

Will "get-Mailbox -Database DB1" work if DB1 is corrupt ?

yes it will because the mailbox database location is stored in AD, so if you run get-mailbox with database condition it will query the AD and find all the users and look for their homemdb attribute and provide you the list of users.

here is a technet article for more info - technet.microsoft.com/.../bb123558(v=exchg.65).aspx