The AT&T USBConnect Mercury is a 1.7Mbps U.S.B. dongle that works with Windows and Macs and is completely 3G HSPA compatible. The dongle will be free with a 2-year $60/month or more data plan. The basic 5GB domestic plan costs $60 for US customers.

Customers traveling outside the U.S. can choose one of AT&T’s
DataConnect North America or DataConnect Global plans. The DataConnect
North America plan includes 100 megabytes (MB) of monthly data use in
Canada and Mexico plus 5 GB of monthly data use in the U.S. for $109.99 per
month. DataConnect Global plans offer discounted data rates in 67 popular
countries plus 5 GB of data use in the U.S. For $139.99 per month customers
receive 100MB of monthly data in those 67 countries, or for $229.99 per
month customers receive 200MB of monthly use in those same 67 countries.

The dongle comes from Sierra Wireless, makers of fine dongles, and should be available now.