Based on what I found I created a helper class AWSRequestSigner, that takes a dictionary of parameters to pass to the webservice, and returns a ready made NSURL object that can be used in an NSURLRequest object as in the following example:

NSURL *url = [AWSRequestSigner urlForParameterDict:[NSDictionary dictionaryWithObjectsAndKeys:
                                               @"Sea Of Tears",        @"Title",
                                               @"Eilen Jewell",        @"Artist",
                                               @"ItemSearch",          @"Operation",
                                               @"Images",              @"ResponseGroup",
                                               @"Music",               @"SearchIndex",
                                               @"AWSECommerceService", @"Service",
                                               nil]];

To look at the source code for the AWSRequestSigner class, download it here and open it in XCode.