Two novel ways to test mobile devices came to my attention today. This has traditionally been a sticky area for developers as you could not exactly point the device to "localhost" and hack away.
xip.io is a service from 37Signals, who bring us Ruby on Rails-based products like Basecamp. At a glance, it's somewhat simple, but pretty cool. You can use the service to point your mobile device towards a local development server, passing through a public DNS server. Be sure to use your internal LAN address (not necessarily the one the public internet sees).
Adobe Shadow is a different take. This Adobe Labs project essentially allows you to mirror your desktop on a mobile device via a small application installed on each. With it, you can synchronously browse your site, viewing both the desktop and mobile experiences.