Scott,
I've asked this same question on a number of occasions, and still awaiting an answer from someone's who's done it, or experienced it.

My wife originally had a multi-entry French-issued Schengen visa, which we used to go to Paris.
Later in the year, we decided to go to Spain, but were unsure (like u) if we could enter into Spain directly from the UK with the same visa.
We decided not to chance it.... and flew into Southern France, then a few days later crossed the border into Spain.

... so... I'm still none the wiser.

Has ANYONE out there actually been in this situation ? (obtaining a Schengen visa from one country, and entering the Schengen area via another)