I'm researching some mapping solutions right now - and frankly they are all below what I really want out of a map!
No standards, no common set's of features. No standard features like a click-able area and I honestly don't trust those API's to stay the way they are and will need sooo much maintenance.
Cool:
Virtual Earth Mobile - http://www.viavirtualearth.com/vve/Gallery/VEMobile.ashx
[ Updated version 1.64 http://blogs.msdn.com/windowsmobile/archive/2006/06/27/649384.aspx ]
Lame:
Google Maps Mobile - http://www.google.com/gmm/index.html