SillySprite, you mentioned flying from London to MSP - Minneapolis is my 'home' airport, and i have to tell you, in years (8 or 9??) of flying back and forth, I've rarely found a cheaper ticket than directly from KLM/NWA. People flying to Minneapolis don't seem to be quite a spoilt for choice as those landing on either of the coasts - or even in Chicago.
We're planning on flying in September, which is now £536! When I checked the same dates 3 weeks ago, it was £100 less. (: Can't imagine what it would be like in July.
I once got an excellent deal from MSP to LHR via AirCanada...I didn't realise when I bought the ticket that they were going through bankruptcy, so it was rather nervewracking, but they still flew!
Good luck to both of us on finding some reasonably priced tickets...
MTA - the price of £536 was for flying from Birmingham (where I live) to MSP. Just checked KLM's price from Heathrow to MSP in September, and it's £444. July is higher at £634...