$1 million can buy enough mining hardware to take over the bitcoin network and basically ruin it for everybody. Assuming you can even find that many GPUs, which you won't on the open market.

You would need about 3,500 mining rigs, each with 3x6990 AMD cards. I haven't done the maths, but you can probably buy that for $1 million. Edit: More like $5 million.

That's not technically a DDoS attack though.