I have an Exchange Server 2010 running in a virtual machine on our VMWare Esx Server, and we do backups of the virtual machines regulary.
I heard somewhere that the Exchange Server should be stopped during the backup, because else I would risk a data loss. Is this true? Wouldn't this also imply that Exchange Server is not tolerant to a server power loss?
EDIT: I use VMWare ESX 3.5, and therefore do not have the vStorage API.
Exchange is based on a variation of the Jet database, and does not need to be stopped in order to be backed up (in fact, it shouldn't be stopped because some important tasks are carried out as part of an "online" backup).
Exchange server backups should be carried out with an "Exchange aware" backup agent - most decent backup software for Windows Servers should include this. This should be done with Exchange online and should be done in addition to any VMWare host or OS level backups you do.
Microsoft have a page about using Windows Server backup here.