Combining Mobile and PC web site
The challenge of creating and maintaining a Mobile and PC web site is to come up with a solution which allows for a flexible design targeting the different requirements of PC and Mobile, without duplicating your data and efforts.
If you employ different URLs for your Mobile site / PC site (such as m.example.com and www.example.com), you can automatically redirect your users to the correct page. This is relatively easy for Japanese handsets. See CarrierDetection for more details.
The advantages for this are mainly user comfort. A user isn't required to remember two URLs to your service, but can always use one URL.