I have a 17 level psion that has read a Tome of Clear Thought +5. He also has a belt of magnificence, +6 to all abilities. Does the belt increase his bonus power point reservoir or does this fall under the same rule when you don't gain extra skill points from a magic item that gives you a temporary boost as long as you wear it?
I would judge you get the bonus points.
XPH p. 18 says:
Just as a high Intelligence score grants bonus spells to a wizard and a high Wisdom score grants bonus spells to a cleric, a character who manifests powers (psions, psychic warriors, and wilders) gains bonus power points according to his key ability score.
So the rules for bonus spells apply also to bonus power points.
The PHB p. 10 says explicitly:
Several magic items improve ability scores as long as the character is using them.
Some paragraphs below it is explicitly said:
When an ability score changes, all attributes associated with that score change accordingly.
Dirctly after that there is a reference to bonus spells (though not based on a temporary change):
For example, when Mialee becomes a 4thlevel wizard, she decides to increase her Intelligence score to 16. That score gives her a 3rd-level bonus spell (which she’ll pick up upon attaining 5th level, when she becomes able to cast 3rd-level spells)
Raising the ability score with a magic item is just one of the possibilities noted in this rule and there is no exception noted.
The rule you mentioned applies only to advancement and skill points.
PHB, p. 58
Use your character’scurrent Intelligence score, including all permanent changes (such asinherent bonuses, ability drains, or an Intelligence increase gained at step 4, above) but not any temporary changes (such as ability damage, or enhancement bonuses gained from spells or magic items, such as a headband of intellect), to determine the number of skill points you gain.
This exception is meant to prevent having to keep track of "bonus skill points" that have to be deleted once the temporary Int boost ends.
But bonus spells as well as bonus power points are awarded when raising the key ability.