Ericsson just use Sony Branding, they wanted to create a "Playstation" branded phone to rival games on the iPhone but Sony said no.
Very wrong actually.
Sony-Ericsson is a total stand alone company with a 50:50 split of ownership between Sony and Ericsson.
Ericsson provide the mobile chip sets (as they do to most mobile manufacturers) and Sony the add-ons such as cyber shot camera etc.
I stand corrected 
If its such an equal split why did Sony not like the idea of a Playstation phone? :-/
And risk impacting thier PSP market?
PSP does not make calls.
They do it with their music? 'Sony Ericsson Walkman' phones, that impacts their walkman / mp3 players, so why not the same with Playstation? The size of the phone screen and processing power would never match that of the PSP Go
I think we all agree that it makes sense to the consumer. But 2 things:
a) you try dealing with Sony's gaming group, you'll understand
b) Playstation is definately purely Sony, always will be. Not Sony Ericsson.