I agree with with those who note the danger of progressives attacking individual mandates in the health care bill, and how such attacks could be construed to be a criticism of all compulsory policies. That's the conservative attack on mandates, of course, just like it is the conservative attack on taxes paid into social programs - namely, that anything the government requires individuals to do is unacceptable (tellingly, conservatives rarely ever argue it's not acceptable to force people to pay taxes that go to fund, say, the Pentagon).
But here's the difference on health insurance mandates that (as Adam says) progressives should internalize: It's perfectly OK to be against them if there is zero choice of a public option. if the Lieberman-gutted health care bill becomes law, it will be the first time in history a federal law will mandate that you buy a product from a private corporation as an obligation of being alive.* Social Security, Medicare and other social programs are different - they are public systems.
I think that's really the key point - and why the public option has to be connected ideologically to the mandate. A mandate without a public option is different than paying taxes to the government for something like Medicare - it is, instead, being compelled to pay taxes, almost literally, to a private corporation.
I certainly appreciate the point about progressives not wanting to oppose all things compulsory because we don't want to trample our message of funding public priorities for the common good. But I don't think it's bad for progressives to oppose laws that force the public to pay into something the public doesn't own, without giving the public a choice - even a meek one - to pay into something public.
I'm comfortable devoting some of my hard earned money to an entity I have a partial ownership stake in. Sure, my "ownership" stake is not huge nor particularly accountable - I'm one of 300 million people in this country, and the government that administers my ownership stake is insulated from genuine public control by campaign contributions, lobbying, etc. But at least I can have some modicum of direct influence over it through the ballot box.
You can't say the same thing about private corporations - those are dictatorships. And when those dictatorships exist in monopoly situations, as it does in the health insurance market, the customer-company relationship is a king-serf relationship. Forcing individuals to give those dictatorships and kings money without even a choice of a public institution the individual partially owns is immoral.
If saying that means I'm making temporary common cause with libertarians who make a far more extreme argument about all compulsory programs, then that's fine, because I don't believe progressives should concede conservatives a monopoly on issues of personal freedom - especially when faced with a health care proposal whose mandate-sans-public-option is so immoral.
* Yes, you do have to buy other private products like, say, car or homeowners insurance. But, you technically have a choice there - you don't have to own a car or a house. This is the first time you will be forced to buy a private product as a consequence of just being alive.