No, CIS requests DOS to authorize a visa number right before a case gets approved (i.e., everything is clear including security check, and PD is current). CIS did not follow this process this late June (they apparently "pre-requested" visa numbers) hence that fiasco. But basically, till your case is ready, no number is allotted to it.
Excuse my stupid question but was't each I485 already assigned a visa number when he/she first filed?