Jack, the iMac is a sweet computer. if you don't need the portability i'd recommend it. despite what people say, it is upgradeable. just more difficult. but you can upgrade the harddrive, memory, and probably the processor since it is socketed. note that you may invalidate the warranty by doing any of these.
on the laptops: the single mouse button i thought would be annoying, but it really isnt. if you want to right click you hold ctrl + click mouse button. but ive found i right click less with a mac than i need to with windows. you might have to in a program but i havent found this cumbersome. if i'm in Blender then i hook up a mouse cause i need the third button also.
the mouse pad is not difficult to locate when not looking, so i dont know how that even became an issue. plus it's much easier to use. two fingers instead of one will scroll, which i like a lot.
Leopard will be on all the machines. but it wont' be out until sometime in october. so i'd wait. definitely wait. it's going to be awesome.
mac mini might be another way to go if you want to save a lot of money. you said you already have a monitor so that would work out perfect. only thing is memory is maxed at 1GB. but i dont see that as a huge problem. that's what i have in my pro.