a friend of mine showed me the server where he was downloading the sources, that was at a time the PS3 wasn't released yet ( just about to be released) and there were a lot of directories of TVs (cryptic TVs name-numbers, as they usually have) with sources and he showed me quite some of them and every single one had some cell target in the makefile. (I don't say they have spursengines, rather cell chips I'd guess). He is quite a linux freak, so I just blindly believe him, I didn't care at that time bout this at all, as it didn't seem smart to put a cell into a TV. (I think there were no 200hz TVs at that time that would need any processing power).
but to have a better estimate of the cost for a cell or spursengine:
http://www.qj.net/qjnet/playstation-3/isuppli-sony-close-to-breaking-even-with-ps3-slim.html
$37.73 and that was a year ago.
It should be also noted that those 4 processing units were just for illustration, the external addon might have far less or far more.
I also think that the spursengine is some Toshiba product and not really accessible by Sony (although they started this joint venture, I think they're not really 'friends' anymore). I think the "ppu" -ip is hold completely by IBM. So if sony would need to develop something new, they have the freedom to do it in whatever way they want. btw. the 1.6GHz spursengine card was taking 20W if I remember right, dunno if that's too much for an external device (or you'll need cooling etc.)
I would think it's also quite possible that they release some devices with SPUs included and those SPUs can execute custom software. But in contrast to usual "flashing" with drivers, they could dynamically execute some user-space binaries that would have seamless access to memory.
Sony might also have quite some not-usable cells, maybe ones that don't reach 3.2Ghz or with enough working SPUs. recycling them like intel did with celerons might be like free candy. (The spursengine had 4 clocking levels if I remember correct, lowest level was 400MHz I think, the doc is public available in the toshiba spursengine sdk
)