I have read this article on about.com and am aware of the differences in usage and meaning between quand and lorsque. However, in the first type of usage, which in the article is termed temporal correlation, it is said that the two words are essentially interchangeable.
Example of the sort of usage I mean:
Je marchais quand tu m'as téléphoné.
Je marchais lorsque tu m'as téléphoné.
But it has been my experience that two things are never truly identical. There must be some nuance of difference — could someone expand on this?