With Windows 2008 there is no more NTBackup that you can use to backup Exchange 2007 server, and the windows team did not put in support for Exchange streaming backup in the new Windows 2008 backup software.
There is unsupported workarounds with copying DLL’s and use the old NTBackup, but that’s not the recommended way of doing it.
Thankfully Microsoft listened and acted on peoples requests and they will create an addon or whatever its going to be called to the Windows 2008 backup software that can be used for Exchange backups. It will also use the modern way of doing backup with help of the VSS framework.
The sad story is that it’s not downloadable at this moment, but the word is that it will be this summer.