The information is supposed to be published with the
1099 general instructions Hey! they're finally back on-line! Last time I checked (last week), there was still the notice that they had been withdrawn for editing and would be back on June 18th.
Go to page GEN-3, and there's information on Backup Withholding starting in the lower right corner. "...backup withholding at a 28% rate."
I don't see anything about it changing in the next few years like it was going to until this recent change, but haven't had a chance to read through the whole thing.