As AppleInsider reports here, there is a sign; the Mac OS X Leopard will come later. By at least 2 quarters.
Me personally, I believe, Apple does not start the same some as Microsoft with Windows Vista. Which were postponed every quarter by one quarter and finally Microsoft released a product, which is a bit “BETA†so far.
The article contains a quite reasonable marketing and development issue. Apple wants to release the Mac OS X Leopard with the full ability of running MS Windows Vista, which is still an issue to resolve. I am not going to install Vista on my own MacBook Pro as I do not see clear benefits for me, but there are users of Apple, who can run Vista and use the ability of the Mac to run both systems.
Maybe, further development of Paralles could solve this issue better. As I really look forward to seeing Leopard in my Mac.

I don’t see the great improvement over OS 10.4, windows performed a huge overhaul. I can’t say it’s worth it’s $400 pricetag, but I don’t think that mac os is any different. Each new edition costs 100, they’re about even.
Based on Apple’s Track Record, I think they will be true to form and come out with Leopard this Quarter as they have promised…
http://www.switchingtomac.com/