But the big question is, how much validation do you put on the web page and how much do you put in the business objects??? The whole point of OOP is to reuse code, not duplicate it.
Unfortunately, that's one of the problems when dealing with web development. If you're developing for windows you can apply this without any problem because of the stateful environment - you can subcribe to event and validation based on those events. Although it's not recommended that you make specific calls to the bizrules you can because it's all within the same environment and platform.
However, when you'r talking about web development you not only have the stateless environment to deal with you also have the problem of dealing with multiple platforms. Set aside the fact that you may have a different OS for the client but whether it supports clientside scripting. Add another layer of complexity and try to determine what version of the specific client side script they support.
This is why it's important to put your validation code at your business layer in case client validation fails. It's not incorrect to put validation code at the client level since it'll make the user experience much smoother but in the case where it fails you at least have a back up...
