NVidia can only concentrate on the market they're in. They currently have a lot invested in the GPU-based streaming market and other GPU-based systems that have yet to pay off.
The market is really tense right now as well. AMD is struggling as Intel's CPU designs are pushing them out of the market. Many different companies are designing Mobile Chips (Apple, NVidia, and several others).
Intel is coming out with Haswell next year. The mobile version is still an x86 CPU but they figured out how to make it so power efficient that it's now possible to have a TABLET or Transformer (tablet/laptop) with an x86 CPU and therefore run Windows 8, not Windows 8 RT so it will be compatible with all the Windows software.
No. They will not get into the desktop / laptop CPU segment.
The cost to enter that arena is too high and they will need to deal with several years of losses to build up their CPU brand name and attempt to get manufacturers to use their CPUs. Additionally, the desktop and laptop market segments are shrinking / slowing down. Does that make good business sense to you?
nVidia is perfectly happy being part of the SoC business for tablets and smartphones. Both of which are growing market segments. However, they need to improve their SoC integration solution because Qualcomm is far ahead of them.
I never understood why Nvidia never paired up with Intel like Radeon did with AMD. Ironically, I think Radeon is the glue still holding AMD together, since their GPUs still can compete (and outpace in some instances) Nvidia. I read somewhere once that Nvidia was AMD's first choice (not sure if its true), but something happened and they ended up buying Radeon instead. AMD and Radeon would trully fail I think if Intel teamed with Nvida.
You can see why though what jaduarskx pointed out about the cost to enter a new market, Intel currently is brutally embarrassed in terms of on-die graphics performance vs AMD. Say what you will about their processors, plenty of warranted criticism to go around, but Intel is years away from catching AMD's graphics. GPUs and CPUs are both microprocessors, but they're different animals in terms of design. Even a CPU company as great as Intel who has been in the business for many years is a baby in the GPU specialty.