Here's the applicable portion of the statute:

(b) Release or Detention Pending Appeal by the Defendant.—

(1) Except as provided in paragraph (2), the judicial officer shall order that a person who has been found guilty of an offense and sentenced to a term of imprisonment, and who has filed an appeal or a petition for a writ of certiorari, be detained, unless the judicial officer finds—

.....(B) that the appeal is not for the purpose of delay and raises a substantial question of law or fact likely to result in—
(i) reversal,
(ii) an order for a new trial,
(iii) a sentence that does not include a term of imprisonment, or
(iv) a reduced sentence to a term of imprisonment less than the total of the time already served plus the expected duration of the appeal process.

On the voluntary surrender issue, from the Sentencing Transcript (sorry for the caps but that's how it appears in the original):

WELLS: MR. FITZGERALD TOLD ME IT WAS HIS POSITION THAT HE DID NOT SEE MR. LIBBY TO BE A FLIGHT RISK. HE SAID HE WOULD AGREE TO A REASONABLE SURRENDER DATE AND THAT HE WAS NOT GOING TO BE ASKING TODAY THAT HE BE REMANDED. I TOLD HIM IN LIGHT OF THAT --

THE COURT: WELL, I MEAN I GUESS I DON'T HAVE A PROBLEM. I WAS GOING TO PERMIT HIM TO SELF-SURRENDER ANYHOW. SO IT WOULD BE SIX-TO-EIGHT WEEKS, I ASSUME, BEFORE THE BUREAU OF PRISONS WOULD DESIGNATE A FACILITY.