Actually, there are some paladin attack powers that also mark the target, including some (e.g. arcing smite) that can mark multiple opponents. These are specific powers, though; paladins do not get the fighter's "mark everything you attack" ability.
However, since these marks are not created with divine challenge, all they do to the targets is impose the marked condition: -2 to attacks that don't include the paladin. They can't trigger radiant damage if the marked critter ignores the paladin, because that's a property of divine challenge, not an innate feature of the paladin's marks.
Although, come to think of it, that might make a pretty cool paladin class feat.
I was thinking of making a feat like that. My players are a gestalt paladin/invoker and rogue/warlock. The paladin dosent seek very good at defending as it has no way of a actually stopping monsters.
how quickly people dismiss the paladin as a defender. There are many attacks that give paladins extra marks. There are also many attacks that essentially say "No you didn't hit my friend you hit me/yourself."