This post in the Omni Group's forum clarifies the differences between buying their applications direct from the web site or through the new Mac App Store. It's particularly relevant for existing owners of their products (and, presumably, products from other developers). It also covers the interesting question of when and why the Mac App Store can sometimes show the INSTALLED button if you purchased an identical build recently from the direct web site while, in most cases, the existing installation is not recognised and you see the BUY NOW button. Read the full post here.