A normal USB port is only 550mA, so yes that will be fine.
Im pretty sure that the back of the Zune says
up to 1.4A it will charge from, and it needs at least 550mA to initiate a charge.
However, whether it will work is still a bit iffy, some adapters work... some don't. I started talking about it here...
http://www.zuneboards.com/component/...8/topic,1132.0