Are ambidextrous people better at multitasking? I'm strongly 'one-handed' in that I can barely even write with my left hand. My wife is fairly ambidextrous, in that she's by default left-handed, but can also write with her right hand. 

I've noticed that she's capable of driving in the dark with full, stressful concentration and still be able to talk. Whereas I'd have trouble driving in the dark and holding a conversation at the same time.

I've heard that this is because one handed people have developed a side of the brain more strongly than the other, whereas ambidextrous people have developed both. Anything that implies that ambidexterity is a disadvantage for multitasking would also be helpful.
This might not answer the question well enough to count as a good answer, but if you're interested in handedness in general, I highly recommend Stanley Coren's well-written and well-researched "The Left-Hander Syndrome."

If I recall correctly, the book doesn't address multitasking specifically, but it does address many issues associated with handedness, including lateralization of language and identity, development of mood disorders, and behavioral differences (especially risk of injury) between left- and right-handers. You might get some good pointers by reading the references.