XHTML Mobile Profile offers some advantages over the original WML, particularly in the area of content presentation. One of the most important benefits comes with the WAP Cascading Style Sheets (WCSS) which provides an easy way to optimize same XHTML content to different platforms. The content itself needs to be created only once and by creating different style sheets optimized for different devices and screens, the same content can be easily displayed across the whole device category.
For further information about XHTML issues, please see Nokia XHTML/WML section at the Forum Nokia web site. It includes a lot of documentation, such as XHTML guidelines, XHTML White Paper, Migration from WML to XHTML, FAQs etc.