I think the FBI will keep it, especially if the trial leads to a conviction ("drug money" and other seized assets are kept by the government upon conviction, providing a nice revenue stream that many say leads to overzealous enforcement of drug laws when the local police agency is feeling a little poor).

It could provide the USA with a storehouse of Bitcoin, in much the same way as Fort Knox provides a storehouse of physical gold.  If Bitcoin takes off in the future and becomes more widely accepted, this could prove quite useful to the government as an asset.