Since I discussed it earlier here are some research studies on it.

Stroke following chiropractic manipulation of the cervical spine

http://www.springerlink.com/content/gyvw3wralxb15van/

Risk of Vertebrobasilar Stroke and Chiropractic Care
http://www.ncbi.nlm.nih.gov/pmc/articles/pmc2271108/
Chiropractic Stroke Awareness Group
http://www.chiropracticstroke.com/

The negative view:
Chiropractic's Dirty Secret:
Neck Manipulation and Strokes
http://www.quackwatch.com/01QuackeryRelatedTopics/chirostroke.html

The industries excuse or positive view:
Chiropractic and the Risk of Stroke
http://www.worldchiropracticalliance.org/positions/stroke.htm

I don't care how statistically insignificant the chances are, they are not worth the risk.
This is all my opinion so ask your doctor about it(not your chiropractor).