LHR access remains the nub of the problem. Slots are not available at transatlatic timings from the normal slot pool (ie, the slots that are given away for free). This means that the only source of slots is from other airlines. The price is fairly high though - GBP 10m to GBP 12m per daily slot pair for prime morning slots, maybe set to go higher in a sellers market.
But who would sell? BA and BD will need theirs for their own Open Skies aspirations. CO/DL/NW could always look to their Skyteam partners AF/KL/AZ but they don't have too many slots that they could give up without making whole routes like CD-LHR or AMS-LHR unviable. Maybe 4 slot pairs in total - max.
Other than alliance partners, potential sellers of slots get hard to find.
Even if the runway slots can be found, coverting shorthaul/small aircaft services into longhaul/big airrcaft services is not automatic. Before T5 open in 2008 terminal and stand capacity is very limited. BA and BD face this problem as well.
So, Open Skies doesn't necessarily mean that much for LHR in practice.