I think the question on where and why the dice are so low is already pretty well answered, but I wanted to offer a couple of thoughts on the other questions.
They're a low-supply item and the Dresden Files release suddenly brought the dice to a somewhat larger audience than had previously been buying the dice. I checked every vendor at GenCon and every single one was sold and out and made some variant on the "I wish we'd brought a lot more of those" when asked.
Fudge, Fate, and Dresden are all minor variants on the same system, but they 'spin' differently.
Spirit of the Century is a *wonderful* pulp game that I have had a lot of fun with over the years. It's kind of cinematic and over-the-top (IMO) and works really well for the old serials and Indiana Jones-style games.
Dresden, being a modern game with a very large supernatural element, works differently despite being the same rules-set. It seems a bit less forgiving in combat, but that could just be because magic is pretty nasty.
Both games are awesome and the real difference is what you want to run with them.
I'd like to note in passing that I have a fledgling game of Dresden Files running now with a bunch of players that have never read the books (only my wife and I have read them) -- I just showed them the template section of the book and let them go nuts. They get to discover the world nearly as much as any non-savvy mortal and it's worked really well so far. Granted, it's less than a month old, but so far, so good...
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Now if only I could get some more dice for it!
2) Is Dresden Files really that good of a game that it's causing dice shortages? Or are they just a low supply item in general already?
They're a low-supply item and the Dresden Files release suddenly brought the dice to a somewhat larger audience than had previously been buying the dice. I checked every vendor at GenCon and every single one was sold and out and made some variant on the "I wish we'd brought a lot more of those" when asked.
3) Should I still even bother with Spirit of the Century, if they both run essentially the same game system? Will that book add much? I've never played Dresden, and only played SotC at cons (but loved it).
Fudge, Fate, and Dresden are all minor variants on the same system, but they 'spin' differently.
Spirit of the Century is a *wonderful* pulp game that I have had a lot of fun with over the years. It's kind of cinematic and over-the-top (IMO) and works really well for the old serials and Indiana Jones-style games.
Dresden, being a modern game with a very large supernatural element, works differently despite being the same rules-set. It seems a bit less forgiving in combat, but that could just be because magic is pretty nasty.

Both games are awesome and the real difference is what you want to run with them.
I'd like to note in passing that I have a fledgling game of Dresden Files running now with a bunch of players that have never read the books (only my wife and I have read them) -- I just showed them the template section of the book and let them go nuts. They get to discover the world nearly as much as any non-savvy mortal and it's worked really well so far. Granted, it's less than a month old, but so far, so good...

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Now if only I could get some more dice for it!