Service Pack 2 (SP2) for Exchange Server 2007 will finally add a plug-in that will enable Exchange backups through the Windows Server 2008 Backup tool. But while SP2 isn't publicly available, if you really, really want to backup Exchange Server 2007 (and maybe save a couple thousand bucks in backup licenses), there is an unsupported way of doing it using the good old NT Backup.
For this we need a Windows 2003 x64 server (preferably R2 with SP2 *and* Exchange 2007 installed), from which we'll copy the following files, from C:\Windows\System32:
Drop them anywhere you want on the Windows Server 2008 box (I sugest creating a new folder called "NTBackup" under C:\Program Files), and you are ready to start doing some online backups.
The NT Backup utility will work as expected and you'll have the option to backup the Exchange Information Store. If you use the schedule option, be sure to modify the command line of the scheduled task (use Task Scheduler), since it will point to the original location, C:\Windows\System32, and not the new one, C:\Program Files\NTBackup.