The desired slot policy from the EU is slot trading and there is a lot of work going on, on how it's to be done. Slot trading is the most economical and most effective way of allocating slots, it will sure come in the future (finally we'll get rid of all these stupid grandfather rights).
US Airlines have to buy slots at LHR, like all other airlines. The FAA already announced it is open to the discussion, as long as the treatment is the same for all airlines.
With the Open Sky Agreement, I'm sure we'll see other US airlines looking for slots and, with no other chance, buying them.
vc10: excellent statements!
BTW: There is a veery interesting study on slot allocation, done by NERA (london based research institute) for the EU. Don't have the link, but it's worth reading it (about 400 pages)