If you are running SharePoint Server 2010 Enterprise, you can use InfoPath 2010 to modify SharePoint list forms. You can take advantage of all that InfoPath has to offer (conditional formatting, validation, etc.) when customizing the list form. This video shows how you can modify the out of the box Tasks list in SharePoint using InfoPath.
This video is part of the InfoPath 2010 video tutorials DVD
Course: SP203 – InfoPath 2010: Design Electronic SharePoint Forms