As you may know, the controversy on wether H. neanderthalensis and H. sapiens mixed on the arrival of the latter to West Asia and/or Europe did not die with the sampling of Neanderthal mtDNA that happened to be totally out of range with modern humans (while Cro-Magnons were fully in line, even if their haplogroup was somewhat exotic). A lot of people, including many scientists remain convinced that there must have existed some inter-specific sex with offsprings sufficiently viable as to produce introgression of Neanderthal genes into modern humans.
The problem is that none of such genes has been found and only theoretical models seem to support the introgression hypothesis. Last year it was discovered that, as some had fantasized, some Neanderthals had red hair... but the genetic mutation causing it was (again) totally different from that of modern humans.
Once and again the hybridist field has failed to provide any conclusive evidence of introgression, even if that has been found to be common in other species.
In Octuber 2007, J. K. Wall and S. K. Kim published in PLOS Genetics a research titled: Inconsistencies in Neanderthal Genomic DNA Sequences. It deals with two divergent studies on Neanderthal genetics, one suggesting no admixture and the other suggesting important hybridation. The authors conclude that the second study's results were only due to contamination of the Neandethal DNA and that the true Neanderthal admixture rate is 0.