This post will probably raise a few hackles but I came across this article about "ADHD myths" which propagates a few myths of its own.
One of these is the claim that ADHD is just as common among girls and boys.
Now ADHD may be under-diagnosed in girls but doesn't mean the percentage of boys and girls with ADHD are similar for each subtype of ADHD.
In the case of "classic" hyperactive/impulsive ADHD there are definitely more boys than girls with the disorder. You just don't see as many girls with extreme impulsivity or hyperactivity, and with good reason - the parts of the frontal lobes of the brain which control behaviour are usually more developed in girls and boys.
In the case ADHD combined type the gender gap is narrower. While there are probably still more boys than girls have this form of the disorder, it is under-diagnosed in girls and often more difficult to spot. For example, while a boy with ADHD combined type may be physically hyperactive (the most obvious indication of ADHD) a girl may be a "chatty cathy" who talks excessively and is overly dramatic.
In the case of inattentive ADHD, the number of girls and boys with this subtype are about the same. Again, this makes sense from a developmental point of view - inattentive ADHD is not a behavioural disorder like hyperactive/impuslive ADHD, so it doesn't make scientific sense for there to be a big gender difference.
Here's a good article describing how the different subtypes of ADHD tend to appear among girls.