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The iPad, at least, has a slide switch on the side which allows you to hold the screen in either orientation.

You can also lock orientation on iPhone in the menu that appears with the triple-press of home button.

With Android you have countless options to fix the orientation. Use the settings menu, use a one-click widget on your desktops, use a button in the notification pulldown menu, etc. You could even easily make it stop rotating if you use certain apps by using Llama or a similar tool. Same for making that by time of day or else.

If you mean physical orientation, then well, I just lay it in a way that it's supported by pillow or bed wall to keep it vertical.

Also, I like sleeping with my tablet because I don't have to get up from bed to turn it on and check e-mail & HN ;).

I did mean physical orientation, but I see from rereading my question that's not clear at all.

Both my Android tablet (Nexus 7) and phone (Galaxy S2) have a software rotation-lock button.