The unfortunate answer is that it's doubtful that any numeric answer exists for this, but there are lots of examples out there. Some places I would suggest you look to get a sense of what's going on:
- Judy Brown's mLearnopedia or the associated content community for mobile learning.
- Blog summary of a recent presentation by Judy Brown
- Lucy Carter's presentation about Sales Training at Apple
If anyone knows a source that will give us an idea of "how much this is happening" - that would be great.