Thanks to Mauricio, I found this comprehensive article on corporate smartphone and PDA corporate security. And since the author, Japp van Ekris, is a Microsoft Most Valuable Professional, it comes as no surprise that he focuses on Windows Mobile devices:

PDAs, and especially smartphones, are very popular devices. Sadly many IT departments still ignore Windows Mobile devices. The devices live in the grey area where they do interact with the business systems (like Exchange and SharePoint), but are not secured according to company policy. This results in an unmanaged risk in the infrastructure: the devices do contain business information but no demands are posed upon them to make them secure. We identify the risks that are introduced and show you that these risks are in fact unnecessary because you can do reduce the risks without adding costs or overhead. In this article we show how you can achieve a reasonable level of security that has minimal impact on user experience and budget.

If you are in a company that relies on smartphones and PDAs to improve productivity, as well as important secrets that need protecting from competitors, you should check out the article. Even if it's Windows Mobile-centric, the policies van Ekris suggests apply to mobiles and handhelds in general.