Sure, each of the networks has a USB modem for wireless broadband, but most of them are hideous dongle things that just don't suit the Air's pleasing, minimalist design. Not to mention that they aren't exactly convenient for me working on the train on my way into the office.
So that's probably why I like the look of 3's new USB "Internet Key" for mobile broadband. It plugs straight into a USB slot and away you go.
It can deliver speeds of up to 1.5Mbps and is HSDPA 7.2 enabled, and works with both PCs and Macs. It's $10 a month, plus data (either $15/month for 1GB, $29/month for 3GB or $49/month for 6GB) on a 24 month plan, or $5 per month for existing 3 customers and people willing to get a bit less bang for their buck ($29/month for 2GB or $49/month for 4GB) over 24 months.
Of course, you need to remember that 3 has some issues with coverage (the price doesn't include data outside of their 3G network), but it looks like this is the price - and product - to beat when it comes to mobile broadband.