I recently acquired an expired domain. On further examination, I found that it was orginally the official website of a company that used to be listed in NASDAQ. That company was acquired by a European company in 2002, and in turn acquired by a well-known multi-national in 2005. I would if there could be any potential legal problems down the line if I use this domain?
One correction to a statement above. domain names CAN NOT be trademarked (I tried using a lawyer and the USPTO rejected my application). Now domain names can contain nonsensical phrases (e.g. "Google") that can be trademarked. So if the domain name is a collection of dictionary words with a TLD you are probably okay.
As others have said stay clear of anything that might confuse your activities with the old company AND don't do anything to capitalize on the domain's last intended purpose.