Just so you know you are replying to a post from 2008. I did this when I first started here too. Just don't want you to waste your time replying to posts that are long since dead.
Scalp itching can be caused by a reaction to shampoos that you buy at a grocery store that contain sodium laurel sulphate. Try shampoos form a health food store like the Aubrey brand.
Yeah I constantly itch too and I can barely get decent sleep because of it. It's to the point of where I scratch until I bleed and wake up with scratch marks all over my body because I scratch so much while I sleep. So now I have beautiful scars on my back and stomach because of it :(. I'm on Lamictal and Adderall xr. It's really frustrating trying to sleep at night. I'm going to go to the doctors again soon so I'll post back more info that may help you. I think this itching thing isn't a symptom of bp though because when I typed in google there were barely any results for it.
Medications can certainly be the culprit but having Bipolar I don't think would cause those symtoms. I would agree with slim351 and see your doctor.
Good luck!!
What medications are you on? Because they can certainly cause the symptoms you are describing. It would be best to discuss this with your doctor. The bipolar alone will not cause that.