If I knock a creature prone and also manage to immobilize it (or if I prone an immobilized creature of I immobilize a prone creature) can it stand up on its next turn?
Put simple: Can I stand up from prone if I'm immobilized?
Yes, a prone immobilized creature can stand up.
The definition for Immobilized could seem a little fuzzy upon first reading:
When a creature is immobilized, it can’t move, unless it teleports or is pulled, pushed, or slid.
Until you note that Move is explicitly defined:
Any instance of movement, whether it is done willingly or unwillingly. Whenever a creature, an object, or an effect leaves a square to enter another, it is moving. Shifting, teleporting, and being pushed are all examples of moves.
An easy way to sum it up would be to say that the creature cannot leave it's square by it's own actions while immobilized.
The Rules compendium states this more clearly than previous rules sources:
An immobilized creature can’t reposition itself on the battle grid, but it’s not paralyzed. It can still stand up when it’s prone... (RC 232)
If you decided to rule that being immobilized prevented creatures from moving at all then you should probably also rule that an immobilized creature can't:
I should note that there are no conditions that explicitly prevent a creature from standing up. Neither Dazed or Restrained will prevent it. Conditions like Stunned, or Helpless would do the trick, but only because they prevent the creature from taking any action at all.
I don't agree with Pat Ludwig here.
You cannot stand up if you are prone and immobilized!
From the way I read it:
1) Standing up = move action.
2) Immobilize = no move actions except teleport, push, pull or slide.
Let me break down the 'Move' confusion. If every square represents 5ft then every PC that is above 5ft is technically shifting into another square when they stand up. With this in mind, standing up is considered a shift.
Standing Up = Shifting
I say this because if you look at what provokes Opportunity Attacks, you will see that move actions provoke Opportunity Attack with the exception of shifting. If standing up is a move action without provoking an Opportunity Action then it counts as a shift.