So can you load the 496 software onto your ARM board ?

There is nothing sacred about the 496's software. As I said, any
given month I hand A complex menu user interface to one of our mid-
level programmers and expect something cool back in three days to a
week (Far in excess of what you would need in a mobile device- Smooth
scrolling, live sprite elements, etc), all switchable so that we can
hit the full range of handsets without difficulty (and designed with
relative references to work on any screen size from 96x74 on up)

The 'hard' part is the GPS interface, and even that is growing more
and more trivial, with SiRF and others now offering integration
directly into an onboard embedded operating system with a runtime
library (rather than formerly having to process the analog signal to
digital and parse the data yourself).

Just go to your local electronics shop and see how trivially available
in-car GPS's are now, from every manufacturer... The technology is
_NO_ different (not even more reliable).

Hell, the 'truly' hard part for any device I would want to build would
be the 3d engine, and even those are coming available for some of the
wider-supported embedded platforms... If not, I'm good friends with
the gentleman who did the 3d engine for Commanche and several other
mid-90s products, so worse comes to worse I could do my own. (I'm
very interested in 'virtual-forward' views and perspective terrain).

The map data itself is fairly trivial... a bit of licensing expense,
that's all.

It actually strikes me more and more that, I will bet you dollars to
donuts that the only reason Lawrence and Avmap don't have XM weather
is an exclusive contract with Garmin that Garmin is paying good money
to maintain... (that's how things work in my industry, its not about
the technology, its about the Licenses... its not what do you make,
its who do you have). Why take the risk on designing a new project
when for a stipend per month you can have the whole market to
yourself?

I have _NO_ evidence of this of course... But its standard fare
business practice in my industry, and frankly I would be surprised if
that _wasn't_ the case.