The congruent number problem is the problem of figuring out whether a given positive integer $N$ is the area of a right-angled triangle with all side lengths rational. According to Dickson's "History of the theory of numbers" (volume 2 chapter 16) the problem was explicitly raised in an anonymous Arab manuscript which dates back to 972AD or before. Some relatively straightforward algebra shows that $N$ is a congruent number iff the elliptic curve $E_N:=y^2=x^3-N^2x$ has positive (algebraic) rank.

Tunnell in 1983 gave a relatively efficient algorithm for computing the special value of the $L$-function of $E_N$ at 1 (I don't know how efficient it is in practice but it's certainly going to run in time polynomial in $N$ which I know is atrocious if $N$ is very large, however I think our problems are going to start before then). If the value of this $L$-function at 1 is non-zero then by the Coates-Wiles theorem the curve has algebraic rank zero and hence $N$ is not congruent. If BSD is true then Tunnell's algorithm gives a necessary and sufficient condition for $N$ to be congruent.

However BSD is not yet known, and my guess is that as $N$ increases, the fact that we don't know BSD for $E_N$ will trouble us before Tunnell's algorithm starts slowing up. In theory there could exist some $N$ for which $E_N$ had analytic rank 2 or more, and algebraic rank zero. Tunnell would compute the special value at 1 to be zero and then we would be faced with the problem of computing the algebraic rank, which can very occasionally be hard in practice, especially if Sha is non-trivial etc etc. Even if the algebraic rank is positive, finding a non-torsion point can be hard in practice; the question is like Pell where relatively innocuous values of $N$ like 157 can give rise to elliptic curves for which the height of every non-torsion point is very large. In fact general theory can deal with 157 because the analytic rank is 1, but certainly the analytic rank can be higher than 1 in general (for example if $N=41$ then the analytic rank is 2).

Because of this issue, I think that (until a new major advance is made with this problem) there must surely be some explicit smallest positive integer $N$ for which we don't know whether or not $N$ is a congruent number. My guess is that this value of $N$ will actually be a congruent number in practice, and the elliptic curve $E_N$ will be computationally checked to have analytic rank at least 2, but no-one will be able to find a rational point (not because there isn't one, but because it's out of computational range).

My guess is that there will be papers containing results of the form "we have solved the congruent number problem for all $N\leq 10^{10}$" or some such statement -- but I have no idea what the state of the art is, and perhaps my google-fu is weak today. This does not change the fact that I am giving a talk on the congruent number problem tomorrow :-) and I thought that it might be nice to mention the state of the art, if I can find it out by then! Does anyone have any leads?