FWIW, I just got my first bill. I bought my device through AT&T's site, using the "Buy with no contract" option, which indicated that the activation fee was $0.00. I paid $99.99 for the Mobley, and signed up for $20/mo unlimited data service on the site at the same time.
True to that, my bill has a $25 charge for activation, followed by a $25 credit. Total bill $23 and change. It appears that includes a dollar and change that's listed as a one time fee for adding the data package, so long term it should be ~$22.
I think this even beats buying the device through Best Buy for $80, then paying the activation fee. Plus, it was ready to use right out of the box from AT&T, so no need to mess with trying to get it activated and get service.
So far, this device rocks. I'm using it to work from home at least two days a week, and it's had zero connection issues.