Yes, except for the faster update, EVERYTHING ELSE
could be provided to owners of a 396 via software and be
put on the data card. Wanna bet they don't offer it?

I'm sure they won't, but I'm also not sure that you get all that on a
data card and access it at a reasonable rate. The 496 must have quite a
lot more memory than the 396, and processing that data will take
horsepower, too.

Well, the street maps for the 396 are already available on CD for
installation on the data card. The 496 just has the internal memory to
store them without a data card. Garmin basically just "threw in" the
auto kit. Taxiways are nothing more than "streets", so they could easily
be put on a card. The AOPA Directory is mixed text and graphics that are
the same as other text/graphics that the 396 already displays. Add a tab
to the airport listing page and pull up the AOPA data and taxiways. A
1Gb card should do it all for a couple of major metro areas, the
taxiways, and the AOPA directory. May even need less. Someone would have
to do the math.

The data itself for the taxiways and the AOPA directory may exist
outside the Garmin walls. Not sure what the license would cost or if
they gave Garmin an exclusive for the taxiway coordinates/data and the
AOPA data. The cards are also available from an alternate source (or you
could just buy one from Garmin). The rest is getting the data formatted
for the 396 and tweaking their firmware. Not a monumental chore. You are
flirting with the license prohibition against reverse engineering. If
you don't sell it or give it away... well... The biggest headache would
be updating your firmware patch whenever Garmin put out an update that
stomped on it. The usual problem when you mess with factory code.

Not sure what the "real" data card is inside those Garmin proprietary
wrappers, but I'm sure the speed of today's cards should be plenty fast
enough. They must be because the auto kit essentially already does this
(street moving maps via the data card). How fast does it need to be to
display a moving map of a taxiway when moving at 10kts?

Sounds like an interesting project. I would rather Garmin did it for a
reasonable price though.

Good Luck,
Mike