Recent attacks on government sites:
Cyber attackers target South Korea and US (www.guardian.co.uk)
All software has holes in it. It's all hackable. The question is, do you want to make it easy for hackers to hack your site or difficult?
Make sure all software running your website is up to date. This includes your CMS, blogging software, scripts used for feedback forms (which was one of the first hacks I ever saw), e-commerce / shopping cart software, forums, ad management software and so on.
If you apply the available patches and do upgrades, then you're less of a target. You're blocking known holes. If you neglect that, then watch out - it's just a matter of time.