Mobile stylesheets and mobile browser detection is all well and good. We can all agree it is easier to implement, and it's better than nothing. But in 2009 it is simply not enough if you want a high success rate with users.
What's the problem? Well, there are 2:
1. Mobile phone networks (for the most part) are not fast enough. Although you can make a site look great with a mobile stylesheet, it still takes seemingly forever to download the full page content as if it were being displayed in a computer browser.
2. Not everyone has an unlimited data plan. Don't force them to download 250kb of data when all they need to see is 25kb of it. Many mobile users still pay by the kilobyte or megabyte.
In Jacob Nielsen's recent article about mobile web usability in 2009, he makes the case for having a separate mobile website all together. He lists the reasons above, along with some other compelling information collected from their usability tests.
If you are about to take on a mobile website project, you should seriously consider the benefits of building a 100% separate version. Your audience will thank you!