Certainly having 50 million plus printers in existence is impressive.
Perhaps I'm not creative enough, but I'm having trouble thinking of consumer apps that people will actually buy and use on their printer.
Sure, I can think of free apps that will encourage printing and make money for HP. But I don't work for HP anymore, so I'm not going to write them. I also know their is a good market for enterprise app related to (usually larger) printers.
In any case, there would little point in HP saying "Write an app once and it will run both on Palm Phones and these DeskJet printers". I don't think people will want the same apps for both.