A properly layered application will generally use DTO's to prevent business entities from being adulterated and distorted to suit the needs of other layers, among other reasons.
However, in very small apps you could spare yourself the burden of DTO mapping and have the business entities passed all the way to the UI. You can keep them in the BLL, it's not dramatic if all layers have a reference to it. It is not really a layered application any more anyway since you sort of crunched the architecture into one big layer.
Mark Seemann has an interesting blog post on this problem.