The open source Nokia is looking to adopt is Linux, already seen on the N810. Nokia seems to be making the move to cut development costs.
Nokia also doesn't just sell phones, they also sell network infrastructure and service and software outside of the telecomms industry.
Quite why they think that the OSS world will help them with DRM is beyond me. It seems highly optimistic at best. But since it is open source they can really do what they want provided they are willing to comply with the license and pay their own developers. There is not much stopping companies erecting DRM on top of Linux (and other GPLv2 code) - see TiVo and the motivation for moving to GPL v3 for an example.
A friend had hanged out with some Ericsson developers he knows and they were laughing at the iphone design. "It hasn't got any high performance mobile chips", "They're designing this phone totally wrong"... Yeah, it's because the phone is no longer a phone, it's a computer. (Hopefully a computer one can install a free OS on and liberate :)