I understand the legal point, but I don't think incorporating should be a barrier to building another application. I wouldn't be afraid of pursuing different web 2.0 apps under the same company, but would definitely consider a spin off if one became successful.
The key points are (a) when one interest starts to run in the red, you don't want it taking down your other interests; and (b) when your app takes off, you don't want to have to worry about app scaling and availability and converting from a sole proprietorship to a corporation or spinning off a company at the same time.
