Likely 2 very, very different markets either company are shooting for, as John said. The RaspberryPi folks are doing a teaching board IMHO, where as the nVidia guys are most certainly shooting for something a lot more mainstream. Doubt either product will step on each others toes in the least.
I was at a talk by one of their founder recently.
Rasberry PI is produced by a charity, not a company. They did it to get kids interested in programming. If someone else comes along that does the same it's not competition - they'll actually be happy about it.