dusky - 5:30 am on May 14, 2010 (gmt 0)
A lot of domains which are about to expire get renewed within the grace period, which I think is two months, but it's worth contacting the owner like what buckworks said, and preferably buy the whole site with its content intact.
Some people may sell the site and keep the content to upload on another domain, it's not that good to you in the long run even if you build content similar to what the backlinks are referencing, to start with it's not the exact content people recommended, secondly, it's an entirely new content from SEs perspective. Even if you construct pages with the exact URLs the backlinks are pointing to, the content is different no matter how similar or close to the original and linkers are not going to be very happy. However, there is certainly more value in it than the bare domain, if you redirect traffic to a hub page explaining and users make their mind up, you gain new backlinks and loose some, likely also the hub redirect page may gain good rank itself to pass on to the homepage and PR snowballs from there to your new content pages.
I may as well throw in this little story.
I bought an expired domain three years ago, I did not know it was a full blown site until I checked the Internet archive, luckily most of its old pages were there. I constructed the same page URLs and wrote my own content, it did not have a lot of backlinks, but at least when people came through those backlinks, they did not get a 404, never had a complaint as the content was about a piece of software the site was selling which I inherited with the site (abandoned software) from which all links were pointing at the site and the domain name was the copyright holder, so technically I am now the copyright holder anyway.
I tried contacting the original owner countless times, no answer, he just vanished, I needed him to work with me to update the software (a windows .exe program) to which I did not have the source code. I ended up recompiling it as it is after stumbling on a version from someone bought before which had no time limit, added to it some new FAQ and links to pages on the site, only in the compiled HTNL pages (did not manage to get to the source) all in a compressed .exe file along with the original program, and I am offering it as free software. I only wanted to buy the domain and ended up with the software as well as the site. I came to the conclusion that the previous owner (from China) died in the Earth quake that hit china at that time, hope I am wrong and he'll contact me one day, but it's three years now.