Officially it's Ctrl+F11 & Ctrl+F12 or KEYPAD 7 & KEYPAD 9.

In practise it's a bit quirky.

  1. Specifically it's Left Ctrl+F11 and Left Ctrl+F12 to switch to previous orientation and next orientation respectively.
  2. You have to release Ctrl before you can rotate again.
  3. KEYPAD 7 and KEYPAD 9 only work with Num Lock OFF (so they're acting as Home & PageUp rather than 7 & 9).
  4. The only orientations are vertically upright and rotated one quarter-turn anti-clockwise.

Maybe a bit too much info for such a simple question, but it drove me half-mad finding this out.