I got this question the other day, and it was a first for me. I had no idea. So, after getting some assistance from some friends at corporate, I have attained some enlightenment. The Windows Mobile platform has the same 5+5 year plan that our other O/S platforms have. (Here's a complete list of products.)
But, to further complicate this, our OEM's can have a different plan - since they are the first line of support for our device customers. So, even though we provide the dates below, your OEM can shorten these; therefore, check with your OEM to be sure.
WM5 Support Lifecycle (2005/Oct 2010/Oct 2015)
WM 2003 SE Support Lifecycle (2004/July 2009/July 2014)
Interestingly, WM6 is not on this list yet.