[eluser]steelaz[/eluser]
I was searching forums about this today. There are a lot of good extensions to validation class, but if all you need is to make sure that checkbox (or at least one checkbox in array) is checked, you have to add two rules:
Code:
$rules['chkArray'] = 'required|isset';
"isset" rule is undocumented, not sure why.
I hope someone will find this useful.