Ofcourse there's more to an Enterprise Mobile Solutions discussion than just TCO, but in many discussions TCO is an important driver. I do question by the way how many companies actually still make the business case for mobile email (regardless if its RIM or Microsoft). The paper referred to in this post compares the RIM BlackBerry TCO against a Windows Mobile solution.
The paper is sponsored by Microsoft, however it is the first recent paper I've seen, let me know if you know of other TCO reports ...
This white paper is a comparative lab-based study of mobile platforms benchmarking the Microsoft Windows Mobile 6 Messaging and Security Feature Pack coupled with Exchange Server 2003 SP2 solution against the RIM Blackberry Handheld 4.x and Blackberry Enterprise Server 4.x.
By: Wipro Product Strategy and Architecture Practice, June 2007
Update 6/7/07 : As many people have indicated the document is only available in .docx format (Word 2007). I have put a .PDF file on my blog for those who have not yet upgraded or installed the compatibility pack ;-).
Download the PDF version of the report : here