I thought I'd check a Pali commentary to see if there was any
explanation for the difference of eta.m and eso in the formula and
found something in an old .tiikaa (Ps-p.t I 286 (Be)) that seems to
help clarify. From my search of the formula in the Suttantapi.taka,
the first occurence is found at M I 40 in the Sallekhasutta. The
a.t.thakathaa (Ps I 182-3) devotes about half a page to explaining the
formula including an explanation similar to Nina's earlier
explanation. I will focus primarily on the middle clause (nesohamasmi)
of the formula because it is the easiest one to deal with.
For "nesohamasmi", the a.t.thakathaa (Ps I 182) gives "ahampi eso na
asmi" which doesn't explain, at least to me, the change from eta.m in
the first clause to eso in the middle one. Now, the .tiikaa expands on
this with the following comment:
Ahampi eso na asmiiti eso pañcakkhandhapabhedo ahampi na asmi,
ahanti so gahetabbo na hotiiti attho. (Ps-p.t I 286 (Be))
I would translate "eso pañcakkhandhapabhedo ahampi na asmi," as "this
I, too, consisting of the five aggregates does not exist,". For the
last part, which is not so clear, I would suggest "the meaning is that
this "I" is not to be grasped." or perhaps "the meaning of 'aha.m'
is that it should not be grasped". So from these comments, one could
translate "nesohamasmi" as "this I does not exist" which is quite
different from saying "this I am not". For "eta.m" in the first
clause, the same .tiikaa gives "eta.m khandhapa~ncaka.m".