I think they cannot afford to spend a dime to develop a new system for a few more years. Look at their financials. Their game division is in the black for 4 consecutive years and lost more than 4 billion dollars in total. You have to make some money first to burry more on a new system. Even if they start to make profit this year I don't see how they can develop their next generation system soon, which would make them lose more. Frankly, IMO PS4 before 2013/2014 seems impossible.
Microsoft is even in worst situation, because they are 6b $ below the line as a xbox division.
And even if they're having profits [300-400m$ per year], that's just too slow for getting to 0$ balance before next gen [probably 2013]. Which will, of course, definitely sink more money from division.
Sony started showing profits too [since Slim] and they are still over the line [300-400m$ as company not division though]. Sony's Playstation is prepared better for next-gen, because they've got Cell already, and its cheap for produce [and will be even cheaper soon - 32nm] , they've got blu-ray which also is cheap to produce and Nvidia after RSX failure probably has some commitments to Sony.
Even if Sony goes with simple road in getting 2x Cell/Cell2, 2gb DDR5 and mid-end fermi from 2012 - PS4 will still be at least 6-7 more powerfull than PS3 and it will be probably cost about 300$ to produce - quite cheap for starting machine [and they dont have to worry about loss on every console].
With great SPE libraries like MLAA, depth of field, motion blur, triangle occuling, EDGE, PhyreEngine and many multiplatform engines behind [Frostbite, CryEngine 3, UE 3] they wont have problems with 3rd party developers.
From Microsoft perspective they need to catch up in terms of performance and in terms of disc capacity, so they have to spend money [that they dont have as a division] for a research.
I think Microsoft's weapon for surviving, as a xbox brand, is putting all resources to promote Kinect to take some of cake from Wii's casual market and those enormous incomes Nintendo has this gen.
If they'll manage to increase profits from xbox division to about 800m-1.2b $ per years - we will see, hardcore and high performance console from Microsoft, otherwise they'll probably go Wii's route to get those 6b $ out of a hole.