Of course there are some limits to this, most notably that you don't see the 3D world. Functionally you can do lot, for example chat, teleport, pay and tranfer inventory. This is usefull if you need to login to SL from a computer that doesn't has or can't run the Viewer.
To bring this functionality to even more devices and locations a iPhone version has been released. Though this version does not has as much functionality as the web app yet. So far you can Chat, send IMs to your your friends List and recieve IMs. This is a cool and quick way to say hi to a friend or answer a customer while you are on the road, or when you are at a boring movie for example.
To use it you can browse with your iPhone or iPod touch to http://ajaxlife.net . Or if you don't have a iPhone you can use http://iphonetester.com/ (seems to work best in Safari) and browse to ajaxlife.net/?iphone in it.
Pictures of AjaxLife via the iphone tester
There was some trouble with posting this and had to redo it 2 times, my apologies for the repeats in your rss readers.