When you move site collection from one content database to another content database in Sharepoint V3, you use stsadm -o mergecontentdbs command line.
If you have enabled auditing on the site collection that you'd like to move to the new content database, mergecontentdbs does move the auditdata table if your SharePoint V3 farm is at least on December 2010 Cumulative Update (12.0.6550.5002). If your farm is not at December 2010 CU yet, your auditdata table won't be carried over to the new content database when you perform mergecontentdbs.
Here's the excerpt related to this issue from the list of Issues that this hotfix (December 2010 CU package) fixes ,
For more detail:
Description of the Windows SharePoint Services 3.0 hotfix package (Sts-x-none.msp): December 30, 2010
SharePoint 2010 does move auditdata to the new content database when you perform mergecontentdbs/move-spsite.