Subscription means that, as long as it is active, you can get new releases of the subscribed product without paying anything more (at least this is the way it works for Infinite). E.g., I payed little more than 100 Euros in february to renew the subscription and I will get 10 Infinite without additional payment if it is released before Feb 2012 (which is almost certain, considering the usual before-the-new-version E-On offers).

I don't know about other versions, but with the subscription I have access to .5 intermediate releases which might not be available otherwise.

As far as dates are concerned, E-On has been quite reliable in the last years, releasing a major version every two years and a .5 in the in-betweeen years, usually during winter.

GIMP 2.7.4, Inkscape 0.48, Genetica 3.6 Basic, FilterForge 3 Professional, Blender 2.61, SketchUp 8, PoserPro 2012, Vue 10 Infinite, World Machine 2.3, GeoControl 2