How to stub a class method in OCMock?

Update for OCMock 3

OCMock has modernized its syntax for supporting class method stubbing:

id classMock = OCMClassMock([SomeClass class]);
OCMStub(ClassMethod([classMock aMethod])).andReturn(aValue);

Update

OCMock now supports class method stubbing out of the box. The OP’s code should now work as posted. If there is an instance method with the same name as the class method, the syntax is:

[[[[mock stub] classMethod] andReturn:aValue] aMethod]

See OCMock’s Features.

Original Answer

Sample code following Barry Wark’s answer.

The fake class, just stubbing connectionWithRequest:delegate:

@interface FakeNSURLConnection : NSURLConnection
+ (id)sharedInstance;
+ (void)setSharedInstance:(id)sharedInstance;
+ (NSURLConnection *)connectionWithRequest:(NSURLRequest *)request delegate:(id<NSURLConnectionDelegate>)delegate;
- (NSURLConnection *)connectionWithRequest:(NSURLRequest *)request delegate:(id<NSURLConnectionDelegate>)delegate;
@end
@implementation FakeNSURLConnection
static id _sharedInstance;
+ (id)sharedInstance { if (!_sharedInstance) { _sharedInstance = [self init]; } return _sharedInstance; }
+ (void)setSharedInstance:(id)sharedInstance { _sharedInstance = sharedInstance; }
+ (NSURLConnection *)connectionWithRequest:(NSURLRequest *)request delegate:(id<NSURLConnectionDelegate>)delegate {
    return [FakeNSURLConnection.sharedInstance connectionWithRequest:request delegate:delegate];
}
- (NSURLConnection *)connectionWithRequest:(NSURLRequest *)request delegate:(id<NSURLConnectionDelegate>)delegate { return nil; }
@end

Switching to and from the mock:

{
    ...
    // Create the mock and swap it in
    id nsurlConnectionMock = [OCMockObject niceMockForClass:FakeNSURLConnection.class];
    [FakeNSURLConnection setSharedInstance:nsurlConnectionMock];
    Method urlOriginalMethod = class_getClassMethod(NSURLConnection.class, @selector(connectionWithRequest:delegate:));
    Method urlNewMethod = class_getClassMethod(FakeNSURLConnection.class, @selector(connectionWithRequest:delegate:));
    method_exchangeImplementations(urlOriginalMethod, urlNewMethod);

    [[nsurlConnectionMock expect] connectionWithRequest:OCMOCK_ANY delegate:OCMOCK_ANY];

    ...
    // Make the call which will do the connectionWithRequest:delegate call
    ...

    // Verify
    [nsurlConnectionMock verify];

    // Unmock
    method_exchangeImplementations(urlNewMethod, urlOriginalMethod);
}