What does the “m” prefix on a domain name mean?

My general answer is that any domain name that has an “m.” instead of a “www.”, or in addition to the “www.” prefix, is probably a mobile enhanced site aimed at users who are accessing the site via their iPhone, Blackberry, G1, and so on.
So if you see “http://m.google.com/”, for example, it should be generally the mobile version of the Google site.
But maybe not. There are a number of different exceptions in this situation that are worth mentioning!
First off, many sites now automatically detect what kind of system you’re using when you access their site. For example, go to Amazon.com on your computer and you get the expected Web page. Visit the exact same URL on an iPhone, however, and you get a customized version of the site designed for that specific phone.
It also turns out that if you own a domain name you can configure it so that any subdomain is mapped to anything you’d like, be it a different server, an alias for the same server as the “www.” site, or even a load-balanced system that spreads queries between ten, a hundred or thousands of different computers.
For example, I have set up the subdomain blog.intuitive.com to actually map directly to http://www.intuitive.com/blog/. Click on that link and watch what shows up in your browser’s address bar.
In conclusion, for many sites “m.” in the domain name means that it’s a site intended for or designed for mobile devices, but ya just never know…