Sometimes a user will ask one question in the title, then a completely different question in the body. Sometimes there are good answers to both of the questions, as here. This can be a problem because a passing user who found the question through a general web search may move on to the next search result after reading the body and finding the question doesn't actually match what he was searching for.
The particular question you cite has been discussed before: meta.unix.stackexchange.com/questions/201/…– Gilles 'SO- stop being evil'Nov 9, 2010 at 0:45
change the body to match the title... ( humor only, don't kill me )– StefanNov 9, 2010 at 4:57
I think the asker should be prompted to clarify and make them match. If (s)he doesn't react, and you have enough reputation to edit the post, I think it's a good idea to edit the title to match the question body.
If there are two completely different questions, it is best if they are posted separately. Again, you can prompt the asker to do so. But if both questions have been answered, it's a bit late for splitting.
If the thread has quietened down, I might even edit the title and tags to reflect the topics that the answers cover. This is more debatable; it's a matter of finding the right balance between respecting the original poster's question and making the answers easier to find for the people who would find them useful.