A horizontally paging, but vertically-flowing UICollectionViewFlowLayout. Oddly, gists don't let you enter markdown into the description, so see the comment below for a description of what it does. Make sure you set its flow direction to **vertical**, even though it'll page horizontally.

gistfile1.m

@interface HorizontallyPagingCollectionViewFlowLayout : UICollectionViewFlowLayout

@end

@implementation HorizontallyPagingCollectionViewFlowLayout

- (CGSize) collectionViewContentSize
{
    CGSize superSize = [super collectionViewContentSize];
    CGSize frameSize = self.collectionView.frame.size;
    NSInteger pages = ceilf(superSize.height / frameSize.height);
    if (superSize.height >= frameSize.height) {
        // multipage
        return CGSizeMake(frameSize.width * pages, frameSize.height);
    } else {
        return superSize;
    }
}

- (NSArray*) layoutAttributesForElementsInRect:(CGRect)rect
{
    CGSize frameSize = self.collectionView.frame.size;
    NSInteger minPage = floorf(rect.origin.x / frameSize.width);
    NSInteger maxPage = floorf((rect.origin.x + rect.size.width) / frameSize.width);
    CGRect transformedFrame = CGRectMake(0, minPage * frameSize.height, frameSize.width, frameSize.height + (maxPage * frameSize.height));
    NSArray *attributesList = [super layoutAttributesForElementsInRect: transformedFrame];
    for (UICollectionViewLayoutAttributes *attributes in attributesList) {
        if (attributes.frame.origin.y >= self.collectionView.frame.size.height) {
            CGFloat parentHeight = self.collectionView.frame.size.height;
            NSInteger page = floorf(attributes.frame.origin.y / parentHeight);
            CGRect frame = attributes.frame;
            frame.origin.y -= (page * parentHeight);
            frame.origin.x += (page * self.collectionView.frame.size.width);
            attributes.frame = frame;
        }
    }
    return attributesList;
}

@end

Standard horizontal flow layout gives you:

[a] [d] | [g]
[b] [e] | [h]
[c] [f] | [i]

This flow layout gives you:

[a] [b] | [g] [h]
[c] [d] | [i]
[e] [f] |

Legend:
[x] = Collection view cell
| = Page separator

This results in a similar "paging grid" style to the Springboard.
Make sure you set the Flow Layout direction to Vertical!