Nintendo wouldn't be able to make a phone themselves. A smartphone is more than just a tech toy, it's the center of a lot of people's lives.
When you mess up a phone OS, you are messing with somone's *life*, ie: their job, getting to a first date on time (maps), social networking, etc.
You can't make the same mistakes Nintendo has made with the Wii U OS, look at how angry people got at Apple for a few hiccups with the new iOS. The OS would have to be 100% flawless.
Video game fans actually put up with a lot (our OS won't be ready for another 6 months, our PSN service has been hacked, you have to pay for our XBLA service to use Netflix) ... regular consumers would drag Apple execs into the streets and kill them if they tried that lol.
Maybe a partnership with Google to make a tablet someday I could see happening.
