I can't see the PB's being upgraded much, because from a marketing sense only G5 will do - hence Power, and the size of the iMac indicates how hard it would be to fit a PB with a G5, even the 17" model.*
I can see no disadvantages in the headless Mac, as it would conquer the only territory they don't already have - cheap PC's such as Tiny/Time/Dell. My mum would buy one. I also think they would be less than $500/£300, as that is the right entry point - Conceivably this may not incude a keyboard/mouse, since most PC users would have one... most certain of the rumours IMHO.
I think the flash iPod is a bit weird, because at least in England everyone seems to shun the small flash ones except people who don't know better, and these guys won't buy an ipod Flash because they can get a PC World special for cheaper. - Would be suprised but not massively if this came out.
As for phones - This would be a market where there is a great product already, (Nokia, closely followed by SE), and Apple would struggle to differentiate their product enough, making it a high risk strategy - I reckon they would licence some bits out to Motorola or some such instead...
Just my ideas.
* I may be swayed because I got a PB in november and would be gutted if they upgraded them already, And from a customer satisfaction view, they were last upgraded in April (?) and so less than a year between upgrades would piss off the people who waited for the bugs to be worked out, such as me.