It's just another platform, another player.
Mobile gaming existed before iPhone/iPad/Android, but has definitely gained more attention and users with these new platforms. Still I think there is room for everyone and the desktop games won't disappear.
I use several platforms, from Windows, GNU Linux, OS X and Android, being able to play the same game in all of them is great but it isn't a requirement and I understand that most users don't have more than one operating system, so being able to reach various platforms is a way to get more users but if you put the mobile platforms in the equation then your game will either have different versions, and be a different game altogether, or be very limited since the mobile platform will be dictating the rules, it's the least powerful of the group.
If you think about mobile, you should also consider web games like flash and JavaScript with HTML5, isn't that also an important platform to reach more users?
In the end, I don't think we should care too much about the current hype, it's a fact that Android and iOS have changed the gaming scene (hey, I see my friend's grandmothers playing the iPad like crazy and they never liked games

) but not every game type runs well on these platforms and there will always be a need for desktop games.
Given the above, if you're starting a project you're the only one in the position to say if you should or should support mobile platforms. If you're starting a FPS, then mobile is probably out of the question. If your starting a rewrite of an old game maybe it could have it's place in the mobile world. It really depends on the game you're building.
Another thing to consider is money, though in decline, the iPhone/iPad and Android stores seem to be a nice way to generate revenue for you project and can be used to support development in other areas. I don't do much game developing but considering the amount of libraries and ready to use code and examples, I think you can build a nice game for any mobile platform in a small amount of time, main problem seems to be the assets like images and sounds, if you have those in a desktop game wouldn't it be easier to just port the game to a iPhone/iPad/Android? Most the code you have to change is related to GUI since you'll work with different GUI systems, the inner works of your game would be the same, somethings you can even use the exact same code (C, C++ and Java on the Android for example).
As a developer I take a look at those platforms but I don't think I should focus on them, there are a lot of areas where they fail and there is room for every platform out there. The desktop isn't going to be replaced by a mobile solution.
I don't know much about Unity, I've read some things about it but I never liked the concept/licenses behind it, still it may be may lack of knowledge about the thing. For Android (the one I'm familiar and develop to) there is lots of libraries and engines you can use, AndEngine is one that comes to mind, and you can use OpenGL in it's lighter version OpenGL ES. You either develop in Java or with a mix of Java and C++. Any game that needs some speed will have to be developed in both Java and C++, where you use Java for general GUI and interaction with the Android OS and C++ in lower level things like physics and access to OpenGL ES, you can do it all in Java in most 2D games but you'll need the speed that C++ brings in the Android for other heavier things.
In iOS you will most likely develop in ObjectiveC, there are some other platforms for development but Apple makes it clear that it supports ObjectiveC and all other languages are second or third class citizens. There are game engines for ObjectiveC but most of the ones I know are paid solutions. Also, for developing for iOS you need to be a registered developer and pay the 100$/year subscription (200$ if you want to develop in both App Stores (iPhone and OS X).
If you go the HTML5/JavaScript way, then you'll have your game in all worlds
