> I could have sworn that I read somewhere that Jefferson stated that he
> opposed the imposition of death until it could first be proved to him the
> infallibility of man.
>
> However, now that i am looking, I can only find a quote of that nature by the
> Marquis de Layfayette.
>
> Did I dream this or do you know of such a quote?

I am not familiar with the passage you suggest, and doubt that it was
written by Jefferson.  Although he might favor the abolition of the death
penalty were he alive today, it is fairly certain that he was not completely
opposed to it in his life time, since he wrote:

"Society [has] a right to erase from the roll of its members any one who
rendered his own existence inconsistent with theirs; to withdraw from him
the protection of their laws, and to remove him from among them by exile, or
even by death if necessary."--Thomas Jefferson to L. H. Girardin, 1815. ME
14:277

"The fantastical idea of virtue and the public good being a sufficient
security to the state against the commission of crimes,... was never mine.
It is only the sanguinary hue of our penal laws which I meant to object to.
Punishments I know are necessary, and I would provide them strict and
inflexible, but proportioned to the crime. Death might be inflicted for
murder and perhaps for treason, [but I] would take out of the description of
treason all crimes which are not such in their nature. Rape, buggery, etc.,
punish by castration. All other crimes by working on high roads, rivers,
gallies, etc., a certain time proportioned to the offence... Laws thus
proportionate and mild should never be dispensed with. Let mercy be the
character of the lawgiver, but let the judge be a mere machine. The mercies
of the law will be dispensed equally and impartially to every description of
men; those of the judge or of the executive power will be the eccentric
impulses of whimsical, capricious designing man." --Thomas Jefferson to
Edmund Pendleton, 1776. Papers 1:505

"If the enemy shall put to death, torture or otherwise ill-treat any of the
hostages in their hands,... recourse must be had to retaliation as the sole
means of stopping the progress of human butchery, and... for that purpose
punishments of the same kind and degree [should] be inflicted on an equal
number of their subjects taken by us till they shall be taught due respect
to the violated rights of nations." --Thomas Jefferson: Report to Congress,
1776. Papers 1:403

But Jefferson also wrote:

"Nothing... is unchangeable but the inherent and unalienable rights of
man." --Thomas Jefferson to John Cartwright, 1824. ME 16:48

And he made allowances for the advances of future generations, as in the
following:

"When I contemplate the immense advances in science and discoveries in the
arts which have been made within the period of my life, I look forward with
confidence to equal advances by the present generation, and have no doubt
they will consequently be as much wiser than we have been as we than our
fathers were, and they than the burners of witches." --Thomas Jefferson to
Benjamin Waterhouse, 1818. ME 15:164

For reasons too involved to go into here, I believe that were Jefferson
alive today, he probably would oppose the death penalty.  But I don't
believe he wrote anything that would suggest he felt that way during his
lifetime.