Stanley Porter's blog gives this teaser of a forthcoming essay,
I see Gal 3:28 (within the context of Gal 3:26-29) as confronting three major pillars of the oppressive Roman socio-economic system—ethnicity, slavery, and gender. Paul pulls the rug out from beneath some of these major fixtures within Roman society. Romans saw themselves as privileged as opposed to foreigners. Slaves were the backbone of the economy. Women were not people but property within a rigid and hierarchical family structure. Paul essentially dissolves all of these within the Christian community.
I see further support for this position in a number of other passages. For example, even though Paul recognizes that races exist—hence his strong statements about his being a Jew and the role of ethnic Israel (see Romans 9-11 among others)—he does not believe that race privileges people within the church.
He also attacks the very institution of slavery without calling for a slave revolt, by making the institution unable to continue by transforming social relations within the church. Those who were master and slave are transformed into brothers (see the letter to Philemon, as well as the Haustafeln passages in Ephesians and Colossians).
Paul transforms gender relations also. I think that even such notoriously controversial passages as 1 Corinthians 14:33-36 and 1 Timothy 2:9-15, when they are linguistically understood within their contexts (without reading into them previously decided interpretations), conform to this understanding. What Paul is arguing for is order and propriety within the Christian community, with proper behaviour on all sides.