Yes. It's probably worth highlighting that, regards artistic works, which is what software is, a console as a piece of hardware is in effect a blank piece of paper on which you are not allowed to refuse the artists what they will create. You won't buy a block of water colour papers with the licensing deal on it stating you may only paint pictures approved by the paper's manufacturer. As owner of the console, I am free to produce whatever art I want, or display whatever arts from other people, just like a TV - Samsung can't refuse me access to channels it doesn't like, and if it's FW blocks some channels, I'm allowed to unblock them if I find means.As long as they didn't use any Sony IP(copyrights, trademarks, patents, trade secrets) that they didn't already have a license for or weren't covered under fair use (aka "works on playstation 3"), then yes. In fact anyone could do it for any of the consoles.
The issue is it is pretty hard to do that without knowledge of the hardware, using middleware, etc. They would probably have to firewall a team which would only be able to utilize publicly available information and likely no third party or internally developed middleware.
However, to make money from the consoles, the console companies lock out developers using copyright laws that mean the console company's IP rights have to be respected when producing software, such as having to use Sony codes and encoding software, or documentation of essential systems covered by NDAs. Sony only agree to allow use of their IP for a fee, which is where the current model exists. The moment someone doesn't need to use Sony IP to create software, such as being able to sign their own code, they no longer have to pay Sony anything to publish to PS3. Sony have no software rights whatsoever regard what is run on a PS3. They only have control of what gets published through their mechanisms. It is quite possible and legitimate for an indie store to appear selling XMB apps as long as they aren't infringing Sony's software IP rights.