It seems to me a bit reactive and minimal to see solidarity simply as ability to sympathise with others' pain. Certainly the roots of moral behavior, in terms of child development, can be found in sympathy, imaginative engagement and also altruistic punishment.

But when you turn this into social ethics, it sounds much like bland bourgeois charity. Rorty thinks of himself as man of the left, but surely for leftists, "solidarity" is "Fraternité", as in "Liberté, Égalité, Fraternité, ou la Mort". Solidarity is the basis of equality, not just sympathy. Many people can sympathize with those who work in sweat shops, or are driven from their homes by giant dams (etc.), without really taking seriously their claim to equality.

The same idea of brotherhood in expressed by Paul in his letter to Galatians:
"There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus."

To my mind, these formulations suggest that we should advance each others projects and basic interests as if they were our own: we should share some of our projects and values with them. This kind of socialist solidarity is certainly not for liberals, if liberal means that everybody should just "do their own thing". My own inclination is to accept some such stronger position, and thus reject Rorty's minimalist thinking. (After all, Rorty once called himself a "postmodern bourgeouis liberal", and I certainly don't want to call myself anything like that...)

There are lot's of other things that your blog post made me think, but let's come back to them.