Use a smartphone emulator to test how your site performs on a mobile browser. If you want to specifically target mobile device users, you should create a special mobile site that makes less use of images, multiple columns etc. http://woork.blogspot.com/2008/07/tips-to-design-your-site-for-mobile.html <--- This is the best tutorial I could find on the topic. That post's quite old, but you'll get the idea.
If u want it to be viewable on newer cellphones and smartphones u can do a redirect for mobile users with the user agent but if you want it to be available to ALL cellphone users I would reccomend WAP (http://www.w3schools.com/WAP/wap_intro.asp)
Creating special sites and pages just for mobile is old school thinking but the OP apparently is satisfied so.... And anyone who is still doing browser (user agent) sniffing is a relic from the 90s.
I typically design for all 4 major web browsers, high end mobile, low end mobile. These are three different layouts based on the same database. However, the first web/high-end feature JS while the low end is pure CSS. The reason for this is to optimize the users experience, and not be a lazy webmaster =]
Just one opinion. Can Script that identified the type of browser, and depending on it to send different outputs? It some interesting PHP solution: "http: //detectmobilebrowsers.mobi/"
WAP is probably the best and most efficient route to go when developing content for mobile device users.
For smartphones the standard websites are nearly all fine. My sites can be viewed fine on smartphones, but it would depend on the penetration of smartphones in your target market I suppose. If penetration is low, something like WAP is probably the way to go.