Posting this here so others are aware.
While displaying a Smart Mobile Device Group. If you attempt to export a CSV selecting "Mobile Device" you will always get a .csv of 0 bytes. I confirmed this with support.
Only workaround: Export as "Mobile Device Group Memberships" it will give you multiple duplicate results but you should be able to filter to use excels duplicate removal feature to filter them down.
I hope they fix this soon.
I also found this discussion where something similar looks like it was happening about a year ago but for the computer side of Inventory.
It looks like it was resolved in 10.9