You hear politician and pundits make this claim fairly often in support of a government take over of health care. Leaving that issue aside for now, the question must be asked if the 44,000+ American deaths yearly from a lack of health insurance is true. The answer is almost certainly a no. While the 'true' number may be next to impossible to accurately calculate, the 44k number is very likely a substantial overstatement. Head to http://nooilforpacifists.blogspot.com/2009/10/health-of-america-part-20.html for an extensive breakdown on why that is.
Update: After an email/comment questions; Carl took a stab at figuring out what the real number is. He concluded a three percent difference in mortality between insured and voluntarily uninsured is probably the right number. Beyond that, the difficulty of figuring out the true cause or contribution from all the different causes of increased mortality makes it almost impossible to know.