Arrays instead of join tables

[William Garrison <postgres(at)mobydisk(dot)com>]

I've never worked with a database with arrays, so I'm curious what the 
advantages and disadvantages of using it are.  For example:

-- METHOD 1: The "usual" way --

Items table:
  item_id int,
  item_data1 ...,
  item_data2 ...
  Primary Key = item_id

ItemSet table: <-- Join table
  item_id int,
  set_id int
  Primary Key = (item_id,set_id)
  Foreign Key set_id --> Sets(set_id)
  Foreign Key item_id --> Items(item_id)

Sets table:
  set_id int,
  set_data1 ...,
  set_data2 ...
  Primary Key = set_id

ItemSet is the table joining Items to Sets in a one-to-many 
relationship.  The above is how I would typically set that up in a dbms. 
 But with postgres, I could do this:

-- METHOD 2: Using arrays --

Items table:
  item_id int,
  set_ids int[],  <-- Hey, neato!
  item_data1 ...,
  item_data2 ...,
  Primary Key = item_id

This way I don't even need an ItemSet join table.

+ Efficiency: To return the set_ids for an Item, I could return an array 
back to my C# code instead of a bunch of rows with integers.  That's 
probably faster, right?

- Can't store any additional join info in the ItemSet table, but that's 
okay for my application.

? Can I write a constraint to ensure that set_ids has at least one element?

Is this better or worse?  Can I enforce referential integrity on the 
elements of the set_ids array?  Is it more or less efficient?  What else 
have I missed?