When appearing Tuesday at a closed-door House Intelligence Committee hearing, Bannon revealed he had been ordered by the White House not to talk about any discussions with Trump himself and other key White House figures, but then slipped when he revealed details about the time Trump's son met with Kremlin emisaries inside Trump Tower during the campaign.
He accidentally revealed he discussed the meeting with Trump's first Oval Office chief of staff, Reince Priebus, Trump's first White House spokesperson Sean Spicer, and Presidential attorney Mark Corallo. This error is critically important because it verifies the President's potential obstruction of justice, a felony, in drafting misleading statements about the campaign meeting with the Russians.
Yesterday, media reports also revealed that Bannon will not now be appearing before the grand jury but will be inteviewed by Special Counsel Robert Mueller, who is conducting the criminal investigation into Trump and his associates. He also told Mueller's prosecutors that, unlike when he appeared on Capitol Hill on Tuesday, he will not evoke executive privilege and, instead, will "tell all" (details here).
Having Bannon be interviewed but not testifying strongly suggests he had not previously been cooperating with Mueller's team nor that any plea bargain is in place. The major question, of course, is whether he will provide simply corroborating testimony to validate evidence that Mueller already has, or whether he will be breaking new ground -- or quite possibly both.
Whatever the answer, this is not good news for Trump. The more people from inside his inner circle speak to prosecutors, the greater the possibility of critically damaging evidence emerging.
Time will tell what is the exact situation. Stay tuned.