I think spell resistance applies once per creature when they interact with the spell (i.e. try to use a barred form of dimensional travel in the restricted area).
I think the more important question is does the caster roll once for his check to bypass spell resistance and apply that to all creatures, or roll individual for each. I think the answer is roll once and applies to all creatures (after all you only created one spell effect).
Edit:
Each spell includes an entry that indicates whether spell resistance applies to the spell. In general, whether spell resistance applies depends on what the spell does.
Targeted Spells: Spell resistance applies if the spell is targeted at the creature. Some individually targeted spells can be directed at several creatures simultaneously. In such cases, a creature's spell resistance applies only to the portion of the spell actually targeted at that creature. If several different resistant creatures are subjected to such a spell, each checks its spell resistance separately.
Area Spells: Spell resistance applies if the resistant creature is within the spell's area. It protects the resistant creature without affecting the spell itself.
Effect Spells: Most effect spells summon or create something and are not subject to spell resistance. Sometimes, however, spell resistance applies to effect spells, usually to those that act upon a creature more or less directly, such as web.
Spell resistance can protect a creature from a spell that's already been cast. Check spell resistance when the creature is first affected by the spell.
Check spell resistance only once for any particular casting of a spell or use of a spell-like ability. If spell resistance fails the first time, it fails each time the creature encounters that same casting of the spell. Likewise, if the spell resistance succeeds the first time, it always succeeds. If the creature has voluntarily lowered its spell resistance and is then subjected to a spell, the creature still has a single chance to resist that spell later, when its spell resistance is back up.
Spell resistance has no effect unless the energy created or released by the spell actually goes to work on the resistant creature's mind or body. If the spell acts on anything else and the creature is affected as a consequence, no roll is required. Spell-resistant creatures can be harmed by a spell when they are not being directly affected.
Technically, each creature in the area at the time of casting and each creature entering the area throughout its duration must check SR immediately - they resist, or they don't.
But that gets tedious. Many of them might never try to use any prohibited spells within that area. Some might just be passing through. Others might do whatever they're doing, reading a book, fighting a battle, eating Chinese takeout, whatever, entirely without casting those spells. So there is no need to check SR for each creature immediately, then track their results in case they use one of those spells - just check once the first time they try a prohibited spell and don't bother for anything else.
Claxon, you could argue that either way.
There is no spell resistance unless something with SR enters the area. For example, you could cast Dimensional Lock on an empty room. If you do, would you "check spell resistance", just for the sake of establishing the number for the future if anything does walk in?
I suppose you could, but I don't think anybody does it that way.
So when it says "check spell resistance only once for any particular casting of a spell", it fairly clearly means "once for that particular casting when a creature with SR enters the area".
That said, the wording as it exists does not rule out checking a second time (with a new roll) when a second creature with SR enters, because you're making a "spell resistance check" against each creature's SR.
Arguably, you could use your first roll against everything that enters. The wording here seems to support that, except it says things like "it fails each time the creature enters the same casting" so now it sounds like it's checking per creature, not per spell - or more accurately, per creature per spell.
It boils down to whether you read this as making one roll to establish the permanent value to use against every SR for this spell, or making a "spell resistance check" against each creature that enters. To me, the fact that they keep saying "spell resistance check" makes me think you check once for each creature (and to me "check" means roll, rather than using an established previous roll).
I just seems weird to have different rolls against spell resistance for the same spell.
I think of it as how "hard" the caster is casting the spell. Which would conceivably be the same for a particular instance of a spell.
I do agree checking spell resistance against nothing seems pointless, and I would certainly wait until there is something to check against (otherwise you're just wasting time on something that might not matter).
I also agree that its a little ambiguous and could go either way.