We are beginning to go down the path of mobile browser support for an enterprise e-commerce webapp (Java/Servlet based). Of course there are many decisions to be made, but it seems to me the cornerstone is to be able to reliably detect mobile browsers, and make decisions on the content to be returned accordingly. Is there a standard way to make this determination (quickly) based on the http request, and ideally glean more information about the given browser and device making the request (screen size, html capabilities, etc?).
I would also appreciate any supplemental information that would be of use from someone who has gone down this path of taking an existing large scale enterprise webapp and architect-ing out mobile browser support from the development side.
 I certainly understand the request header and the information about a database of standard user agents is a great help. For those talking about 'other' request header properties, if you could include similar standardized name / resource of values that would be a big help.
 Several users have proposed solutions that involve a call over the wire to some web service that will do the detection. While I'm sure this works, it is not a good solution for an enterprise e-commerce site for two reasons: 1) speed. A call over the wire for every page request to a third party would have huge performance implications. 2) dependency/legal. We'd tie our website response time and key functionality to their service, which is horrible for legal and risk reasons.
@David's answer mentioned using WURFL -- which is probably your best option. Be forewarned, however, the success rate is usually around 60% (from mine and other's experience). With carriers changing UA's constantly and the amount of device profiles that exist (60,000+ ?), there's no bulletproof way to get all the right data you want.
Just a bit of warning before relying heavily on a device DB. I would try to keep the user's options open by allowing them to change session options in case i've guessed wrong.
While you could detect a mobile browser through it's user agent the browser war on the PC platform has shown that sniffing user agents isn't really such a good thing to do.
What ideally should be done is that specific styles should be applied based on media type or that a different answer should be sent based on a header other than the user agent - such as the Accept-header which tells which kind of content that the browser prefers.
Right now it might be enough to code a site that works with the iPhone and with Opera through browser sniffing - but Googles Anroid is coming any minute now and there are many other mobile phones that will have browser functionality close to the iPhone's in the near future and it would be a waste to develop a mobile website that didn't support those devices as good as possibel from scratch.
After days of searching for the right way of detecting a mobile device I've decided to keep it simple [ stupid ] and i shall put a 'Mobile device site' button on my index page.... it's only one click away!!
I propose a free detection system which is based on uaprof and user agent:http://www.mobilemultimedia.be UAprof should be the primary key for detection when it's available as there are usually multiple user agents for the same uaprof. If you want to manage this on your own, you should then go for Wurfl because you can download the entire database and manage it locally by yourself.
When I had a similar need recently, I found this code that uses HTTP_X_WAP_PROFILE, HTTP_ACCEPT, and HTTP_USER_AGENT to identify a browser as mobile or non-mobile. It's PHP but could be converted fairly easily into whatever you need (I implemented it in VBScript for classic ASP).
Ironically, it turned out that I didn't end up using the code because we decided to provide specific URLs for mobile and non-mobile users, but it certainly worked when I was testing it ...