I don't think bipolar disorder is that often diagnosed in childhood. Lots of ADHD diagnosis (which may be what you're thinking of), but the vast majority of the bipolar diagnosis I see are adults.
I will agree though that a lot of the ADHD diagnosis are questionable. Particularly with a study I saw recently that showed a much higher rate of diagnosis amongst the younger half of any given class. When you're in kindergarten or the first few grades, that 1 year possible spread in ages within the class can make a huge difference in maturity, and sometimes people mistake the actions of the child that is simply a bit younger than his or her peers as being out of the ordinary, when in reality they're not really that bad.
CORRECTION: You are right, I'm thinking of ADHD. Sorry about that!
HOWEVER, I have a relative that has been diagnosed with bipolarism. He has supposed blackouts, runs off to a few states away and tells his mom and dad that he had no idea how he got there. YEAH, RIGHT!! He's been to doctors of all types and his parents are convinced that meds will help him. Well, it's been at least seven years and I haven't seen one bit of help. What I have seen is that he is taking FULL advantage of the supposed "illness" and has basically ruled his parent's household. At this point, he is near his mid 20's, he cannot hold a job and lives under his parent's wing as if he was 12 years old. Personally, that's what should have happened when he was 10 or 12 and perhaps he wouldn't be like this today....if you see where I'm going.
If you ask me, people use these medical and psychiatric analyses and terms as things to hide behind because they are too afraid of looking at themselves in the mirror and visualizing and admitting to themselves of their shortcomings. This goes for parents as well as the "patients". What they really need is to get their heads screwed on right via a constructive means.....religion, faith, stay off the cell phone with friends and come back to family, or a good therapist...however they do it, they just need to admit that they have a problem and try to get themselves fixed.
Again, yes, there are some few isolated cases of true bipolarism but I think there has been a bandwagon effect over the past 20 years for the people who are afraid of themselves and they use these "illnesses" as places to hide.