Yes, but I think it is so little that it really might not have an impact at all. Just to compare:
Above was mentioned, that some studies suggest that x-raying babies does have a negative impact. I had a look at average medical x-ray machines: The ones from today are far stronger than my microwave oven, so they translate into an exposure of radiation of several Million mobile phones at once. That does not sound healthy. And I am pretty sure that the 1960s x-ray machines that were in use when the researched people were in fact babies were not more healthy than the ones from today.
So this would mean to me: If x-rays are obviously so much stronger, I would expect everyone who has been x-rayed before to get cancer long before the first hardcore mobile users are affected. But that is just me.