If you cast a silent, still spell, does it completely negate the chance of people finding out that you're casting the spell? What if a caster is looking right at you when the spell is cast? What if they're using detect magic? Would a bluff check help?
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It still provokes an attack of opportunity, just as using a spell-like ability would (or, to put it another way, you're still distracted and offer enemies an opportunity to hit you). People can see that you're concentrating on the target and not taking any visible actions.
A caster couldn't use Spellcraft to identify the spell, since he can't see or hear the spell's components, but I'd allow Spellcraft to oppose a Bluff check.
Detect magic won't help, since the caster won't detect that a new magic aura has appeared until at least his initiative count (that's if there are no other magic auras within his detection cone, otherwise two rounds). Arcane sight might.
A Bluff check is appropriate to pretend that the caster is doing something other than casting. If the caster doesn't particularly draw attention to himself as an attacker (say, if he's in a crowd) opponents might need Perception checks to notice that he's doing anything.