There are two different questions depending on whether the linear extensions are given to us labeled. If we wish to reconstruct a poset $A$ from a labeled list of its linear extensions, then our job is very easy (even if we use "usual" linear extensions with no "=" appearing): for two elements $a, b \in A$, we have $a < b$ in $A$ if and only if $a < b$ in every linear extension of $A$, and so the result follows immediately. (This is the case that Joel David Hamkins addressed a few seconds before I posted :-) .)

If we have unlabeled linear extensions (so really the data that we receive looks like $\{(* < * < *), (* < * = *), (* < * < *)\}$ or the like) then I conjecture that the result is false, with the following potential counter-example.

A Steiner triple system $S$ on a set $X = \{x_1, \ldots, x_{13}\}$ of 13 vertices is a collection of 26 blocks $B_1, \ldots, B_{26}$, each of which is a set of three elements of $X$, such that every two elements of $X$ occur in exactly one block $B_i$. To such a system we may associate a poset $P(S)$ whose vertices are $\{x_1, \ldots, x_{13}, B_1, \ldots, B_{26}\}$ with order relation $\in$. There are two nonisomorphic Steiner triple systems on 13 vertices, yielding two nonisomorphic posets. (Nonisomorphism is easy: the poset conveys exactly the information about which triple contains which elements.) I think that these posets may be a counter-example, for the following (weak) reasons:

Given the list of linear extensions (in your sense) of a poset, it's easy to reconstruct certain basic data about it. In particular, we can find for each $i$ the number of elements whose shortest saturated chain to a minimal (or maximal) element is of length $i$ by looking for the linear extension that is most "bottom-heavy" (or "top-heavy"), in the sense that the initial block of equal elements is as large as possible (which will happen exactly when it consists of all the minimal elements), and the next block of equal elements is as large as possible given that the previous condition is met (which will happen exactly when it consists of all elements that cover a minimal element), and so on. By varying this idea slightly, we can also determine the list of up-degrees of each minimal element in the Hasse diagram (i.e., we know how many elements cover each minimal element), and the list of the number of elements that cover each pair of minimal elements, and so on. This data is not enough to reconstruct the poset on its own, since we don't know how things correspond in the different extensions, and indeed the two posets described above share this data but aren't isomorphic.

Unfortunately, it's clear that knowing all the linear extensions gives us more information than what I've used in the above paragraph. Unfortunately, these posets are too large for me to just slap them into a computer and check whether they really have the same set of extensions. Also, I can't find my copy of Enumerative Combinatorics at the moment to check what is actually known about this sort of thing.

reallyentertain the possibility that the conjecture be not true yet there are no counterexamples to it? $\endgroup$ – Mariano Suárez-Álvarez Nov 8 '10 at 4:06