It has been written on the walls for a while actually. The signs have been there, but ever since Steve Jobs introduced the Apple iPhone running some kind of OS X it became very clear that the mobile only operating systems are going to be dead within a very short time.
The smartphone category is already dominated by Microsoft Windows and Linux is also there. Now Apple sends OS X into the race. In my opinion there is no longer room for or need for a Palm OS, Blackberry OS, or Nokia's Symbian. The hardware got more powerful and the desktop OS developers moved to deliver versions of their operating systems that support mobile usage.
The big advantage of having desktop OS running on your mobile devices is compatibility with your desktop PC at home. You get the same user experience, same file formats and same applications. Another huge factor is the available developer pool for desktop operating systems. There is no need anymore for specialized Blackberry, Palm or Symbian developers. Palm already has Windows powered smartphones (700w and the new 750) in there offering, which was a smart move. Now Palm just needs to be consequent and drop Palm OS.
Nokia is also already making experiences with a Desktop OS. Their Internet Tablets 770 and N800. These are no phones yet, but it is clear that Nokia is thinking about Linux as the OS for their mobile phones.
I did not hear about RIM doing any ventures into desktop operating systems for their mobile devices. From the outside that does not look good.
Microsoft already has several mobile phone makers on their side like Motorola, Samsung and HTC.
Steve Jobs with his Apple iPhone is going to be different at least in the beginning, because they keep OS and Hardware under their control. Apple creates a closed system, but it includes their desktop computers.
Mobile only Operating Systems were necessary back then, now it is time to move on and simplify both the phone maker’s and the consumer’s lives.