If you run the Get-MailboxDatabase commandlet with no switches it returns all the Exchange 2007 databases in the organization. If you are looking for a list of when each database had a full backup you need to use the -Status switch.
Get-MailboxDatabase -Status | Sort -Property LastFullBackup |ft Identity,LastFullBackup
This will return the Identity and the time for the last full backup of each database in sorted order. This is a useful list when doing maintenance in an Exchange organization.
If you need a quick powershell script that dumps each Storage Group and its backup-related information visit the Exchangepedia Blog at: http://exchangepedia.com/blog/2008/09/script-get-storage-group-backup-status.html