You could also show the version number in the the page title. You can set this in your index.html in your theming. This way you can maintain it during each deployment. Of course Fedor's solution is almost the same, but this one doesn't cause database interaction. We do it this way in some of our apps.
Just set the Title tag in your index.html in your deployment folder:
<html> <head> <title>Jan Doggen APP v1.01</title>
You can store this number into an attribute and show it in the header of your style sheets so you don't have to build it into each form. You can maintain this number in runtime accessible by an admin.