The scuttlebutt is that the three companies are interested in working together on something to do with charging electric vehicles, potentially a global recharging standard. Currently, when it comes to fast charging, BMW uses the SAE Combo standard, Nissan uses CHAdeMO and Tesla uses its own Superchargers.
The meeting was prompted by Tesla opening of all of its patents the other day, FT says. Speaking during the recent annual shareholder meeting, Musk said he was "more than happy to have other manufacturers" use Tesla's Superchargers but that, "there is no other electric car that can accept anything close to 135 kilowatts."
Let's assume the technical hurdle can be overcome. With any potential patent/licensing problems out of the way, these discussions with BMW and Nissan might be revolving around this other angle that Musk mentioned: "They (other automakers) just have to contribute to the capital cost, so figure out what percentage of the time are their cars using the Supercharger network and then they can make a contribution proportionate to their customer usage of the Supercharger network." That certainly sounds like something that could take some time to figure out.