What does it mean to be human?

I came accross this article on from BBCnews the other day (Plan to create human-cow embryos), and am disturbed by the implications. If we as humans are able to create a inter-species chimera, where is the line to be drawn? This team proposes to create (and then destroy) embryos, but how long will it be before another group (legitimate or not) proposes to allow development to continue - what then? It seems as though the creations of science-fiction writers were eerily prescient ... how long until we as a race create a Frankenstein or a Cylon? I'm not able ot full formulate my thoughts on this subject quite yet, but it could certainly lead to some deep contemplation about the Big Questions, couldn't it?