I've just upgraded to 2.70 and now when I'm in Front Ortho view (or Back Ortho, Right Ortho, etc) and rotate out of it into User view, it switches out of Orthographic mode into Perspective. Blender 2.69 didn't do this? Why is it happening now in 2.70?
In blender 2.70 Auto Perspective is turned on be default in the user preferences. This will cause you to go from perspective to orthographic when changing from the User View a view like the side view. The same is true visa versa.
To change this behaviour you can edit the user preferences.