I haven't read The Chapter's Due - does it imply or state outright that hybrids are possible? I do hope not.
In the earliest fluff, back when marines were mind-scrubbed convicts, hybrids existed, but they were shoved under the rug afterwards. They don't make sense from a cultural perspective - the eldar see human as animals or vermin, the Imperium see aliens as monsters - and a biological one as the eldar reproductive system is postulated to work in stages. Far, far more importantly, their genetic structure has a triple-helix (which in reality is less stable than a double) and around 20 bases rather than our human 4.
It's impossible for a hybird to occur based on this fact, but frankly GW has never really cared for genetics, or they'd have made some serious changes to the 'nids in Xenology. A transgenic could be artificially created, however - reverse-transcribed eldar genes put into human embryos or vice versa. This wouldn't be a true hybrid, though, it would be a human with eldar genes or an eldar with human genes.
(I'm a Biologist specialising in genetics - I try not to let real science interfere with sci-fi, but damnit sometimes you just can't help yourself...)
Last edited by Eremite; 09-07-10 at 04:00 AM.