Some interesting accounts of how an expired domain does or does not hold it's PR/backlinks if you manage to snap it up quickly enough. Particularly interesting is nmalhot's post early on in the thread:

I have been through 2 PR update cycles and of the 7 the results are as follows:

1 - PR was zero as soon as I put up the site and has yet to go up ... I even linked other sites to this and the PR is stuck at zero. It is a .com domain.
3 - PR dropped 1 notch. 1 is .cc, 1 is .net, 1 is .com.
3 - PR held up. 1 is .com, 1 is .biz, 1 is .org