Well since bitcoins are not really 'money' (not legal tender anyway) yet, taxes wouldn't matter unless you sold it for whatever fiat currency. Other than that can't see what difference is between mining bitcoins and knitting a scarf. )

Sounds great. I'm totally in. I'm gonna be so rich.

Your's

That's the whole point of this exercise.

As long as BFL is giving refunds when asked, you will have a tough time proving your damages in court.  

You could try to argue that you have "consequential or expectancy" (different courts use the terms interchangeably even though they have different meanings), in that if BFL had delivered when they first said they would you would have made more $ because of the lower difficulty.  That argument would likely fail for a dozen reasons though.  

Otherwise you could try to argue that BFL has violated the FTC rules regarding preorders.  But what is that going to get you?  Nominal damages?  (I haven't read the statute to see if the statute defines damages, but I would imagine it doesn't.)  So congratulations.  You just spent tens of thousands of $'s to win $1-$5 in nominal damages.  

So I would wait until you have actually been damaged by BFL before filing suit.  A damage the court would recognize would be if they refused to give you a refund after asking for one.  And this would be easy to prove as well.  

(I didn't completely read OP before posting this)  So after doing so, here's an addendum:

The court is going to laugh you out of the courtroom if you try to argue that you should have received the same amount of BTC in your refund that you sent in.  There are numerous cases dealing with the stock market and currency fluctuations that reject that argument.  You are entitled to the same "value" denominated in USD that you sent in.  You would receive a windfall profit otherwise, and the courts despise giving a plaintiff more than they lost.