Also, make sure ANY AND ALL other software you are running is also kept up-to-date and secure. Rough example - if you are running an outdated version of let's say... hmm OpenX for ads/banners and it is outdated, then yes they can hack that and depending on how they hack it could potentially upload shell scripts or other and then subsequently hack into your forum.
The main thing to note here is: "I own so and so website, I must make sure I keep ALL software up to date, it is my responsibility to know these things and maintain secure software at all times" etc etc