The only real bubble is that companies like Yahoo are willing to pay lots of money for one-app companies with very little tangible value. You should only really be concerned if your plan is to try to make money by creating your own app and selling it to a megacorp.
Where the money is for mobile developers is not making apps themselves, but making apps for businesses that want apps to further their non-app-related goals. It's similar to websites in the 90s - while a few outliers were people making money on websites they were building for themselves, most web developers were making money by building websites for companies to achieve their other business goals.
I've been doing mobile development for over four years now, and this whole time I've been expecting hordes of developers to descend on the market and give me a lot more competition. It doesn't seem to be happening - demand for mobile developers is still far outpacing supply. It will be a good field to be in for years to come. Eventually the mobile developer market will be saturated, but this took a decade for websites, and the people who were any good didn't care because they had the time to build up loads of experience and put themselves at the top of their field.
If you find that you do need to shift your career path, you can generalise quite easily - Java is still widely used in other areas such as server-side web development, and Objective-C will let you write native OS X applications. Generally speaking, if you can handle mobile development, you can handle desktop development with ease.