Steve,

> create procedure spInQuestion
> as
> Begin
> execute procedure spOtherFieldsProcedure

(a) This procedure produces results that are ok.

>
> for select this, that
> from xtable
> into :this, :that
> do
> begin
> execute procedure spDoSomeStuff(:this, :that)
> returning_values :mynum;
>
> update xtargettable
> set FieldThatsZero = :mynum
> where this = :this;

(b) all this struff is incorrect afterwards

> end
> End

So, if I understood you right (see (a) and (b)) then I assume that the
source of the problem must be that procedure (a) does see the data it
should see while the (b) part doesn't.
Since all is running within the same transaction context (in a stored
procedure) it has to be uncommitted data.
But then IBExpert shouldn't see the data either (at least if you execute
while pausing your app with a break line (as you mentioned earlier)).

What transisolation level are you using for your Transactions?

Luc.