This is a great area to be in these days but changes quickly so education and keeping active in the community is essential to stay up to date.
If you can learn these well and get your name out there, it's a great field. Mobile is less clear cut as you have both the web version and the app side... Knowing the later is extremely valuable but a longer learning curve and grasping tweaks for mobile sites [i.e. jquery mobile] is a good start.
Build a few common apps, share some knowledge and you'll find yourself in a field desperate for talent.