For most Mac Pro buyers waiting until March/April for the new model isn't going to be a big problem given that Nehalem will be bringing a lot of muscle and it'll let Apple actually hold a press event for a professional product, something they almost never do.
I've given up on ever seeing a mid-priced desktop/tower from Apple ever again because it would force Apple to negotiate deals on a wider variety of components and, mostly, because Steve Jobs believes computers should be sealed boxes like household appliances. When was the last time you upgraded the heat pump or insulation in your refrigerator? When it's not good enough anymore you're supposed to buy a whole new one. Upgradeable towers run counter to his profit focused world view.
I believe the perfect lineup has 4 notebooks and 4 desktops. I'd replace the Mini with a Nano that uses the same parts as the MacBook to reduce cost and inventory issues. They could charge more because it'd be a legitimate performer and it might actually cost less to produce than the Mini.
I'd also cut down on the standard configurations. I see no point in having three different MacBooks, two 20" iMacs or two MacBook Airs. Just make one model and let people CTO a different processor, HD, RAM, etc.
NetBook
MacNano (same as MacBook minus the display, battery, kb, trackpad)
MacBook
iMac 20"
MacBlade (mythical small desktop/tower)
MacBook Air
iMac 24"
MacBook Pro 15"
MacBook Pro 17"
Mac Pro