If anybody else hits this problem, I figured it out. The problem is, that the nine patch images "btn_toggle_off.9.png" and "btn_toggle_on.9.png" (which you can find in the androids-sdk resources for platform "android-16") contains too small resizable areas (only two separate pixels on the left and top side of the nine patch drawable). Nexus 7 uses the "tvdpi" scale bucket which means it (usually) scales down the hdpi resources. And as you can read on the android developer blog concerning the nexus 7 (i can't paste here the url):
Make sure that any stretchable regions are at least 2x2 pixels in size, else they risk disappearing when scaled down.
Well, it's good to know, that the google guys know how it should be, but unfortunately in this case they just probably forgot to fix these nine patches.
So the workaround is to copy out the resources for toggle button from android-sdk (specifically btn_toggle.xml, btn_toggle_bg.xml and also btn_toggle_off.9.png and btn_toggle_on.9.png for all the scales needed) and to fix the nine patches - make it a pixel larger on the left, top and the right side and enlarge the stretchable regions. Then you can override the default style for toggle buttons and use these resources instead.
Just to mention - this is only a problem if you target some older sdk or if use don't use the new holo themes, where the toggle button looks differently (and the ninepatches are ok).