Here is a presentation on TI Davinci processors.
http://mpsoc-forum.org/2006/slides/Talla.pdfThe 644x also has the H3A system. It also could be used in a camera. It is not as advanced as the DM355.
My wild [admin: avoid swearing please] guess is that the 6445 was Digic II and the DM355 is Digic III. The DM355 just started shipping and Digic III is a relatively new release as well. Coincidence ?