Huber D, Petreanu L, Ghitani N, Ranade S, Hromádka T, Mainen Z, Svoboda K. Sparse optical microstimulation in barrel cortex drives learned behaviour in freely moving mice. Nature. 2008 Jan 3;451(7174):61-4. PubMed.
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The Harvey-Svoboda paper does indeed usefully advance our understanding of the vital topic of the synapse-specificity of LTP. Although the paper does not mention it, other studies have shown that LTP occurs in an all-or-none manner, so that the "threshold-crosstalk" described by these authors is really equivalent to the "LTP-crosstalk" previously described by others (notably, Engert-Bonhoeffer).
However, your summary is slightly wrong on one point: the Harvey-Svoboda evidence that depletion of calcium stores does not affect crosstalk does not rule out that calcium itself might be the "intracellular diffusible factor". This is still a very real possibility. The main argument Harvey-Svoboda advance against calcium is the evidence (in the supplementary material) that in the conditions of their experiments the spread of calcium from synapse to synapse is "only" 1 percent, a number that is not significantly different from zero. However, that number is clearly even less significantly different from 1 percent, a level that could (if repeated 30 times, as in their protocol) combine with sub-threshold calcium signals to trigger synapse-inspecific LTP.
The main, and probably insuperable, difficulty, that synapses encounter in generating completely specific LTP is that they must remain well-coupled electrically to the parent dendrite. While this may seem a minor technicality, it may turn out to be the single most important problem the brain faces, and responsible for much of its baffling circuitry.
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