IMC, a PC's social skills can never be used "against" other PCs. Ever. I mean sure, you can roll it, but it's up to the (target) player to believe, trust, or do what the persuasive PC says... or not. Sense Motive, opposed Diplomacy, or whatever... not required.
With NPCs, consider that skills are less powerful than spells. If a player is expecting charm person style power from skill use, he should be disappointed. If a player is expecting dominate person, he should be upbraided (IMHO) and then disappointed.
[ruleslawyer]
Bluff specifically says it isn't (read: can't reproduce the effects of) even a suggestion spell.* The PHB fluff text for Bluff explains what a "bluff" is: it it quite clearly not a command that the target must follow. The rules text supports this reading: "A successful Bluff check indicates that the target reacts as you wish, at least for a short time (usually 1 round or less) or believes something that you want it to believe." (Emphasis added.) The target is not made to act as the player wishes; the target merely reacts as if the bluff is the truth... usually for 1 round and possibly even less. And remember the +20 modifier to the DC for bluffs that are "almost" too incredible to consider. (A bluff that is too incredible to consider automatically fails IMC.)
* An epic-level character can instill a short-duration suggestion [in an NPC!], but there's a +50 modifier to the Sense Motive roll made to oppose this/set the DC. [EDIT] Note also the suggestion can be detected using Sense Motive. [/EDIT]
Diplomacy specifically says that it can change only attitudes. Attitudes do not force specific behaviors. Even a Helpful NPC won't necessarily take lethal risks, for example.* Don't forget the –10 check modifier for "rush" diplomacy (anything less than 1 full minute; the rushed check is a full-round action). Also don't forget "retries usually do not work."
* An epic-level character can change [an NPC's!] attitude to Fanatic, but the DCs for this are 3 times the DCs for changing to Helpful (changing Helpful to Fanatic is a DC of 50). [EDIT] The Fanatic attitude lasts (1+CHA bonus) days, can be detected with Sense Motive, and can be suppressed or countered (but not dispelled) as though it were a mind-affecting magical effect. [/EDIT]
[/ruleslawyer]