This is true no matter which skill is missing.
If no PC in the group has Perception, does the DM just automatically allow them to perceive ambushes?
No, because there are
explicitly written rules in the book about when you make perception checks to detect ambushes. Or rather, you
don't make checks, because the ambushers generally make stealth checks against your passive perception, but same difference -- there are times written into the book where perception checks are required to avoid a negative outcome.
However, the book does not require the DM to include lurker monsters all the time and base campaign-altering events on perception rolls, so from that perspective it definitely IS up to the DM to decide whether or not to screw the party for their skill selections.
If no PC in the group has History, does the DM just automatically hand out historical documents instead?
Not at all, but it is up to the DM to possibly decide that throwing out a lot of required History rolls is a bad idea when nobody is trained in it. So again, there IS a certain DM choice there, but it's totally off the subject.
It's not a matter of screwing the PCs. It's a matter of the DM have to adjusting the game because the players screwed themselves.
The point I'm making is that the rules do not anywhere require you to have Arcana trained in order to locate or identify magic items, unlike (say) getting Monster Knowledge about dragons, or figuring out what that glowy energy field is for.
The original question was, "Is the party going to be short on magic items if there isn't a high intelligence arcana user?", and my answer is "No, because nothing in the book says you need to have a high arcana check to find or identify magic items, because the book doesn't require that magic items look just like other items before you turn on your mage-o-vision; nor does the book say how long it takes to handle a non-magic items and determine that it is not magical." Therefore, it's up to the DM to decide whether magic items in his game world are obvious or not, and whether or not a few moments of handling is sufficient to determine the presence or absence of magic in an item. Depending on the answers to those questions, a low-Arcana party may or may not be screwed, but it is not required by the rules as written.
In my game, I have two Arcana trained characters, one of whom is a wizard with a massive Int bonus, and I still treat magic items as totally obvious; a magic sword among mundane swords is a Corvette among Toyotas.* But that's just me, and another DM could make another selection however he wants to; I'm not saying it's wrong or right, only that if the party lacks a strong arcana character, the DM's choices could definitely cause problems or avoid them.
* Please don't think too hard about the metaphor. I don't want to hear, "Well,
actually, some of the high end Toyotas..."