Jane Galt, a noted blogger, was getting herself involved in the debate about the uninsured in the US. Wendy had previously weighed in on the subject as well. As a Canadian, it's really easy to tell anecotes about dying children left outside hospital because of lack of insurance, but are things really that bad?
I did a quick and dirty Pubmed search for uninsured cohort studies. I haven't read the actual studies but there does seem to be a substantial body of research on the question of uninsured patients. That uninsured patients receive less care than insured patients seems to be an uncontroversial conclusion.
One thing that I would note is that the lack of insurance is only part of the problem: Medicare/Medicaid is also bad compared to private insurance. This could be seen as a vote of confidence for private insurance and a blow to government-funded care. Remember though that deciding between government and private insurance is a different question than insurance vs. no insurance. When we're sick or hurt, medical care is a need, not a choice, and someone has to pay for it. As Wendy had said, people will get the care, if they show up without money. The U.S. model seems to allow people to get insurance after they get sick, which strikes me as being about as cost-effective as allowing people to get car insurance only after they've had an accident.
From the conclusions of the studies:

We found uninsured children to have consistently poorer quality of care than insured patients.
Socioeconomic status and breast cancer survival were directly associated in the US cohort, but not in the Canadian cohort. Compared with similar patients in Des Moines, residents of the lowest fifth of income areas in Winnipeg experienced a significant 5-year survival advantage (survival rate ratio [SRR] = 1.14). In these lowest income areas, the Canadian survival advantage was larger among women aged 25 to 64 years (SRR = 1.23), and this was observed in the middle fifth of income areas among this younger cohort (SRR = 1.11). The Canadian survival advantage even seemed apparent in the poorest neighborhoods with relatively high representations of Aboriginal people (SRR = 1.16).
Significant differences in types and costs of care were observed across differing insurance types, which may suggest an "insurance effect" on asthma-related treatment in the ED and/or hospital.
Incident cases were more likely than prevalent cases to be women (35.4% vs 15.3%, P =.001), African American (45.4% vs 20.4%, P =.002), and uninsured (29.6% vs 7.6%, P <.001). [AIDS related infections]
The lack of health insurance is associated with an increased risk of a decline in overall health among adults 51 to 61 years old.
Uninsured patients with one of three common chief complaints appear to be less frequently admitted to the hospital than are insured patients, although health status does not appear to be affected. Whether these results reflect underutilization among uninsured patients or overutilization among insured patients remains to be determined.
After adjusting for clinical factors, race and insurance status may independently influence the use of cesarean delivery. The higher rates in nonwhites and lower rates in the uninsured may reflect differences in patient preferences or expectations, differences in physician practice, or unmeasured risk factors. The lower odds of cesarean delivery in uninsured women, particularly women at high risk, may raise the issue of underutilization of services and warrants further study.
Payor status is associated with the use and appropriateness of invasive cardiac procedures but not length of hospital stay after myocardial infarction.
With adjustment for age and income, persons without insurance had higher mortality than those with employer-provided insurance, with relative risks of 1.2 for white men and 1.5 for white women. These relationships held after adjustment for employment status, with the working uninsured showing mortality between 1.2 and 1.3 times higher than that of the working insured.
Lacking health insurance is associated with an increased risk of subsequent mortality, an effect that is evident in all sociodemographic health insurance and mortality groups examined.