Imagine trying to build a smartphone 15 years ago, that would be really hard. You need a decent screen you need a good cpu etc.
All of that is off the shelf now. Anyone can buy it and its cheap. (esp in volume)
In a market in China (I dont remember the name) there are vendors upon vendors who sell all sorts of electrical parts, gadgets etc. You can literally walk down the isles and pick the parts you want in your new smartphone.
So hardware wise, now is a good time. Sure you wont get as polished as the big players right away, but prototypes or a limited set sure.
Now software. There is quite a bit of software that will run on the arm board. With all the hacking on the android platform (sadly a lot more than on the Windows or Apples phones), you can (if you want) at least create a very customized different Android based OS if you want. (would make updates a bitch).
But you can also get some version of linux working and I am sure some other strange os.
But there has never been a time when it was easier to get some dev boards, and start writing the next mobile OS.
Its something you can get started on at home now for less than $1000.
As far as the cloud services and integration that has never been nearly as easy. We have all sorts of providers with stable and "open" APIs.
Providers like MS azure provides a full stack. Database, Data storage, device sync, messaging, computation etc. and they are not the only ones.
I cant think of how much building and setting up all that cloud stuff would have cost in itself. Its out there now, and if you are tesing and writing your app, or have it deployed with low values its very cheap. Sure you wont have your own Music store right away (but licensing is the biggest issue there) but you can piggy back Amazon mp3.
So we have good access to cheap hardware. We have development tools, boards and compilers. We have excellent possibilities cloud sync, integration, etc. We have open source operating systems we can hack at, learn from and understand how things work.
As others have pointed out the future is probably pure data phones. That is a great market to attack right now. Forget the 1984 "phone" part and the nobody really uses anymore SMS.