snipped
So now I need to know how to go about replacing it, so I can continue on
with the deck. I've googled for info without much luck. The little info
I
found via google assumes that the rim joist is perpendicular to the floor
joists, and that it's right over the foundation. In my case, it's
parallel
to the floor joists (and on the gable end of the house), and since it's a
split-entry house, there is a short (approx 5') wall between this joist
and
the block foundation below.

So how long is this wall? The short wall has only a single 2x4 becuase your
rim joist(s?) is bearing the weight. And how long is the total span? And
is this a single joist? Not two 2x12's? Are your floor joists running the
entire width of the house, or do you have posts in the basement?

a) do I have to replace this joist as a complete span, or can I cut it out
and replace the damaged chunk? The posts dealing with the joists right
over
the foundation imply you can just cut out the damaged section, but since
I'm
over a wall, I wasn't sure if this was more structural. For what it's
worth, the wall's top plate is only a single 2x4, not a doubled header
plate
like you'd expect if it were load bearing.
b) What, if anything, do I need to do to brace the house temporarilly
during
this repair? As I have vinyl siding over "build rite", I can get to the
framing reasonably easy from the outside, but both levels of the inside
are
finished and would be a major problem to remove anything down to the
framing
members. The damaged chunk isn't doing anything structurally now, but I
want to make sure I don't have "the walls tumble down" if I remove a
slightly bigger one...

I'm no structural engineer, but my guess is that the rim joist is load
bearing over that short wall. I'm anal, so what I would do is put in new
studs under where you're going to replace the damaged section, just like you
would a header over a new window. So, you're essentially turning a single
load bearing joist/beam into three, with additional studs under the points
where you have to cut the joist, know what I mean? I know this means
removing a potentially large section of wall, but that vinyl siding is easy
to remove and replace. It shouldn't take you long.

What you could do for support is pull off a couple of rows of siding above
the rim joist, then lag bolt a temporary 2x12 to the studs that surround the
area you'll be replacing, then jack up that 2x12 from the outside. make
sense?

Also, when you put on the new deck a good idea is to shim the new ledger out
a bit. Cut some treated 1/4"(or less) strips to use where you'll bolt your
ledger onto the house. Inevitably, water will get behind the deck ledger,
this space will allow for better drainage. Don't forget the new layer of
tar paper over your new rim joist area, then good flashing over the new
ledger as well.

And, of course, you shouldn't listen to any of us. You should call a pro who
can see it with his/her own eyes

Tim