Actually, this is a more general form of the question. Can something that cannot be tripped still be knocked prone from an effect which causes it?
So, in the Gelatinous Cube Stat block, we have:
CMD 9 (can't be tripped)
Now, there are tons of abilities which can knock a creature prone without the Trip Maneuver. For example, here's one of them:
Knockdown (Ex): Once per rage, the barbarian can make a trip attack against one target in place of a melee attack. If successful, the target takes damage equal to the barbarian's Strength modifier and is knocked prone. This does not provoke an attack of opportunity.
So it doesn't seem to make sense that a Gelatinous cube can be knocked prone, because it's an ooze and doesn't seem to care about those kinds of things. However, for some other creatures that can't be tripped it would make sense that they could still be knocked on their butts.
Is there any specific rule that states
(can't be tripped) == immune to being knocked prone? If not, is there something that prevents being knocked prone?
To clarify, I'm only using that power as an example, does this also apply to things that don't say "Make a trip attack" such as the Alchemist's Force Bomb?
Force bomb*: When the alchemist creates a bomb, he can choose to have it inflict force damage. Force bombs deal 1d4 points of force damage, plus 1d4 points of force damage for every odd-numbered level, instead of 1d6. Creatures that take a direct hit from a force bomb are knocked prone unless they succeed on a Reflex save. An alchemist must be at least 8th level before selecting this discovery.