I might be shot down as a heretic for saying this but I'm not sure that the idea of the universal church helps us much. I certainly don't think that the New Testament separates belonging to Christ (and hence the universal church) from relating to a local group of his followers.
All the great baptismal texts in Paul's letters - Galatians 3, 1 Corinthians 12, Romans 6, Colossians 2 - are rooted in local church experience even though they have a cosmic sweep to them.
But I agree with Marcus that we need to get rid of the rules that restrict our membership on any grounds other than discipleship.
I'm just reading Alan Kreider's chapter in Deep church and will comment in due course. I am grateful to Andy for reminding me of Stuart Blythe's description of baptism as a political act - that is something we have lost.
I'd be interested in Andy unpacking his thoughts on having a more theological understanding of membership in relation to covenant. I wonder if this is the category that helps us make the best sense of what membership is and how it works.
If that's the case, then I think what I'm suggesting is that we see membership as a covenant with one another in our churches. Furthermore I'd be interested in exploring ways of regularly renewing that covenant commitment - possibly through an annual covenant service or renewal period (say in January). I know there are one or baptist churches that practice this kind of thing - maybe someone out there can give us chapter and verse.
By bringing the covenant aspect of membership to the fore I think it offers us the chance to have a deeper and richer theology of membership, while at the same time removing some of the unhelpful hurdles we make prospective members jump through before we let them aboard.