Most of the back links are probably dead, or will be if the webmasters of those sites bother to check. If they are directory links, or article links, there is a good chance that they will stay, but nothing is certain. I wouldn't count on it. If you are planning on putting up a site, you could contact the link partners, but, I personally wouldn't keep a link to a parked domain with no content. So, what I am saying is...links from websites are probably going to be gone...links from directories, social bookmarking, articles have a chance of staying active.
whatever backlinks you can find at the time of research before purchase would still be there after you register the dropped domain
I know that people sell registered dropped domains. It they have PR5-7 and backlinks, that costs much
dan from justdropped.com sends out his list of freshly registered dropped domains daily. I haven't bought anything because he usually wants $50-$100+ each but it seems his method gets good results
ALL the backlinks stay in place unless the sites with the back links ON THEM remove them. So buy the domain name and GET A LITTLE content up NOW to ensure all the backlinks stay in place.