Just read a short article in the December issue of the Joomla Community Magazine titled ‘Ten Arguments That Threaten the Security of Your Website’ that is well worth reading. It applies equally well to any web site but obviously the emphasis is upon Joomla.
The point about always keeping your website up to date with the latest patches in particular is one that I usually have to use every week, when looking at reported problems our component users are experiencing. Usually the Joomla version is several if not many versions behind current. Perhaps one might miss or not have time to always be upon the latest release but there is really no valid reason for being, in some cases over a year behind. It is just asking for trouble.
I recommend it for a worth while read, it is reasonably short, but quite concise, and unfortunately so true in many respects.