Providing exhibitors with audited attendance figures definitely is an industry trend. An audit is an independent, census-based confirmation that your verified attendance figures are accurate. ("Verified" means those who actually show up.) Be sure the audit separates out qualified buyers from others (such as exhibitor staff).
Conducting an audit puts you one step up on competitors who count legs (and sometimes arms) when quoting their attendance. You can counter such outlandish claims by saying "prove it, we do." Most exhibitors understand audited figures because they also advertise in magazines, where audits are mandatory.
A typical audit costs about $2400 for a 5000 verified attendee show. These two firms do census-based audits: Expomark (ABCexpomark.com or 847/879-8272) and BPA (bpai.com or 212/779-3200). Expomark is the more experienced and they have been endorsed by IAEM.