First things first, mobile is not dying out. It will be an increasingly large portion of application development for a long time, all of the major players are deeply committed to mobile.
I would not worry about gaining 'unusable' experience. Working in IT/Development means you need to be constantly learning. Gaining experience with the hot platforms (iOS/Android) now will only make it easier to adapt to future mobile advances.
Like I said, the mobile market will only increase in the near to mid future. Anyone entering the mobile space will need to be aware of how to more effectively server their users/customers in a cross-channel and cross-platform way. I believe you can't go wrong with focusing on becoming a mobile expert, if you are passionate about what it offers.
Local Data:
Core Data hooked into a sqlite.db is the way to go. Some say it's a little heavy but I have had a lot of luck with it. It's going to give you everything you need to persist and query data locally on the device.
Remote Data:
You pretty much have to go with a web service layer. The standard now-a-days is RESTful JSON. Directly connecting to a remote DB is too unreliable for the mobile world.
ASIHTTPRequest: A wrapper around network level API's for connecting to web servers. Use this.
I am assuming you are talking about Android/iOS Native Apps. From my experience, the development time for these 2 platforms are essentially the same, assuming the same level of developer for each platform.
The amount of time needed can vary a lot depending on amount of 'polish' required as well how thorough the design and specs are.
With all of that said, I would give an above average developer 2-3 months to develop something like the Yelp app. This is assuming the back-end and web services are ready and available.
I would not worry about gaining 'unusable' experience. Working in IT/Development means you need to be constantly learning. Gaining experience with the hot platforms (iOS/Android) now will only make it easier to adapt to future mobile advances.
Like I said, the mobile market will only increase in the near to mid future. Anyone entering the mobile space will need to be aware of how to more effectively server their users/customers in a cross-channel and cross-platform way. I believe you can't go wrong with focusing on becoming a mobile expert, if you are passionate about what it offers.