They will auction it off as soon as possible like they did several times before.
I'm not sure why the bitcoin price has gone up in response to this news, because it means more supply is coming to the market soonish. These coins might have been presumed lost before. Certainly they would not have been able to be sold on any legitimate exchange. Now they will be washed clean and enter the markets.
On the other hand, a billion dollar bitcoin auction from the US government will generate some publicity, and publicity definitely drives the Bitcoin price up. We'll see which effect dominates.
There's another billion+ hoard of bitcoins which has the potential to be released to the markets in the next year or two, which is the MtGox reserves. Crazy story: MtGox went bankrupt after losing a bunch of bitcoins in hacks, but during the years-long bankruptcy proceedings the bitcoins they have left have been frozen, and their value has gone up so much that they could exit bankruptcy and pay all their debts many times over. But it's unclear what the bankruptcy court will ultimately allow them to do.
If the MtGox bitcoins are released to their many thousands of creditors (MtGox customers who had balances at the time of collapse), I expect a crash in the bitcoin price as many will choose to sell at the same time. This is different from the government bitcoin auctions, where there are only a few winners who probably don't plan to sell right away.