It isn't bad practice. I appreciate when sites do this -- and I like it if they include a link for me to get to the real site if I want.
I found this list in google -- there are other hits if you search for them
- How do I determine a mobile browser
Here's some javascript that will do the job: http://www.quirksmode.org/js/detect.html
- Is it bad practice to have the mobile site on a different domain or subdomain?
It's good practice, and a popular convention is to use http://m.yoursite.com for mobile if your main url is http://yoursite.com
Noah
There are a number of solutions for determining mobile browsers. Most use the UserAgent making the request but some solutions use other factors as well.
Try looking at:
Use http://51degrees.codeplex.com/. It is an ASP.NET open source module which detects mobile devices and provides auto redirection to mobile optimized pages when request is coming from mobile device. It makes use of WURFL mobile device database. For redirection there is no need to modify existing ASP.NET web application pages.