-- Is 64bit Linux/Ubuntu a worthwhile alternative to use as a general use OS?
Absolutely. I love linux in all its flavours. Well... Debian still has me a bit perplexed but I enjoy playing with it nonetheless. Ubuntu surprised me with its simplicity.
-- Does Linux make a good secondary OS? (any dual-boot issues, for instance?)
As mentioned above somewhere, it works great as long as Windows is installed first.
-- What is the future of Microsoft's Windows operating system?
It will continue ahead in leaps and bounds. While there's a small minority heading over to Apple, Linux or whatever, Windows is what new machines come installed with. It will continue to be popular (at least the most widely used), because it's almost everywhere. M$ will have to screw the pooch something awful to lose their market share.
- Will Vista be dominant, or crash and burn against competitors Linux/Mac?
As above....
- Will Windows XP do for another 5 years?
Without a doubt. M$ still supports Win2k and it's 8 years old.
- Is Leopard gonna kick their buts out the desktop market?
Never used a Mac. I saw a quote years ago (dunno who it was) that said "A Mac is a computer with training wheels that can't be taken off". Hence, I've never bought one. 
- How big is the desktop Linux userbase anyway? What gaps in the market has Linux filled?
Pass. No idea.
- How commercial is Linux?
Have a look at the server market. For me, though, I use Windows because it's simple. It does what I want it to.
-- Is Amiga coming back to take them all out? Lol
*cough* I've been waiting (as have most people here) for 15 years to hear something positive. No news as yet. *sigh*
