The blonde woman Desmond sees in the present is Boone and Shannon's babysitter who fell down the stairs. Duh.
Ok, I got that last bit from a friend who (insanely) remembers Boone mentioning that his babysitter Theresa fell down the stairs, back in season 1. This woman is also named Theresa and Faraday presumably did time-displacing experiments with her, which could have caused disorientation and thus the accident.
