The chip is not officially propietary to the raspberry pi foundation but broadcom are notoriously hostile to working with "little guys". The raspberry pi team got special treatment due to a combination of their charitable mission and having people involved who worked for broadcom.
Also the license of the raspberry pi firmware is restricted to use on raspberry pi products, so to remain within the license you would have to make your own firmware builds from the materials broadcom provide (under NDA).
Odroid managed to secure a small batch of BCM2835 and developed a pi-compatible product called the odriod W around it but broadcom refused to sell them further chips. The reason broadcom did this is not clear, the two most plausible reasons I can think of are either that they were violating the license on the raspberry pi foundations firmware or that they couldn't commit to the volumes needed to keep broadcom interested.
Basically if you have a real plan to sell at least hundreds of thousands if not millions of units and you have the capital to back that plan up it may be worth talking to broadcom. Otherwise look elsewhere.
There are many arm based SoCs that can run linux out there with different feature and price levels. Some are more easilly obtained than others.