According to Appleinsider there is suspicion that the real reason for the delay of Leopard 10.5 is related to the secret features more so than the iPhone.
“American Technology Research analyst Shaw Wu told clients on Friday. “We believe the extra time Apple is allocating is for developers to test secret features that will likely be revealed at its WWDC 2007 conference starting on June 11, 2007.” [from Appleinsider]
Another possible consideration is that this would give Apple an opportunity to announce the features at WWDC and then give the developers a chance to react to the new features between June and October.
This piece also appears on Some Mac Blog.