Most economists agree on these two propositions about tax incidence (covered in Chapter 6 of my favorite textbook):
1. It does not matter which side of a market you tax. A tax on buyers is the same as a tax on sellers. In particular, a tax on employers is equivalent to a tax on employees.
2. Because labor demand tends to be more elastic than labor supply, a payroll tax falls largely on employees.
Now consider the Obama health plan. A major element of the plan is an extra payroll tax on firms that do not give their workers health insurance. By the basic theory of tax incidence, this is equivalent to a tax on workers without insurance.
In other words, the Obama plan is much the same as imposing a health insurance mandate, backed up by the penalty of a tax surcharge on your earnings if you fail to have coverage.
One difference: If an individual buys his own health policy, rather than getting it through his employer, he still pays the tax. That is, the Obama policy continues, even reinforces, a strong policy-induced preference for employer-provided over individually-purchased health insurance.
Mankiw goes on to assure that this is just an observation rather than a "don't do it!" I'll go ahead and say that this is a good reason to rethink it. There are already lots of structural problems with tying health insurance to employment.