Rumors are flying, predictions are a dime a dozen... but since this is MY soapbox, here's what I'd bet a latte for:
#1 The 'leak' the other day on the Apple website was Apple's way of testing to see how much the market is paying attention to and invested in their product.
#2 We will see a Powermac similar to the specs posted.
#3 We will see a 'new' 15" Powerbook utilizing the aluminum design of the 12 and 17 inch models. though I suppose that they might just wait a few more weeks to show that to us at a different event. I also don't expect these to have the PPC 970 in them. No company has ever put a 1st generation desktop processor in a laptop, (heat issues/Power draw)
#4 10.3 (aka Panther) will be called something else. It will be shown extensively at the conference, and seeded to developers. It will not ship until the fall. The new boxes will run on an iteration of 10.2.6 that is 64 bit enabled.
#5 A Demonstration of iTunes for windows.
#6 This is where I go out on a limb.... I think we'll see the introduction of a divergent product line. Over the next 6 months we can expect another 'Digital Hub' type device... but we're not going to see it at mondays developers conference. I think what we WILL see is a box positioned directly at the Multi-Media market. Sort of a cross between the top end desktop and the Xserve. This machine will have large internal serial ATA busses, Fiber-Channel and optical digital audio I/O. this is a logical step to take based on the increasing market share of Final Cut Pro, and also Apple's positioning of Logic Audio to take on Pro-Tools. One of the main reasons I believe this is that none of the current hardware lines are 'perfectly' suited for such applications, and Apple would be wise to tailor a box suited to those needs.