Because we are dealing with mobile (battery powered) device, the optimal resolution may indeed push towards multiple of 16s if it enables more efficient decoding with CPU and DSP.

I do not know either details enough, but sounds reasonable to have 448 as a vertical resolution because it is multiple of 16.
http://wiki.maemo.org/Video_encoding

Would be interesting though to get real world data of decoding 800x448 vs 800x450 in N900 and monitor the decoded session with powertop.