A study found that the people that multitask the most are the least effective at it. It would be interesting to evaluate ADHD symptoms in the study sample, and see how it correlates with multitasking effectiveness.
In the MSNBC.com article, Clifford Nass Ph.D. said,
“Is multitasking causing them to be lousy at multitasking, or is their lousiness at multitasking causing them to be multitaskers?” Nass wondered. “Is it born or learned?”
Far beyond me to turn that study into something that makes sense but it looks like it might be what you’re looking for with regards to ADHD symptoms and multitasking effectiveness.