Most companies in the business of mobile monetisation and advertising think if Apple immediately pulled UDIDs, which are used to target advertising and track ads and installs, the effect would be serious.

Of course, Apple's not doing that... yet.

Still, assuming at some point it will withdraw the use of the hardware identifier - as it's been threatening for nine months, perhaps also telling the industry not to use the even less private but as accurate MAC address - if the industry doesn't have good replacements to hand, developers and publishers will stand to lose a lot of money.

Scores on the doors

How much?

Well, respected ad mediation and real-time bidding outfit MoPub has done some basic research.

Based on three months of its data, MoPub reckons publisher inventory (i.e. mobile ads) with UDIDs earned an average eCPM of $0.76, compared to $0.58 cents for ad impressions without any device identification.

The headline figure, then, is the loss of that hardware ID results in a 24 percent drop in revenue, because companies can't tell if you've already seen their ads a hundred times, or track what happens when you click on a banner or video ad.

Worst case scenario

Of course, there are several alternatives being offered to UDIDs and MAC addresses.

For its part, MoPub is using OpenUDID, but other options include the similar SecureUDID, ODIN1, plus various behavioural fingerprinting technologies and web-based cookies.

MoPub hasn't announced any comparative work in terms of how the use of these different technologies would mitigate that 24 percent drop.

It says those technologies haven't been available for long enough.

However, it does mention that it's "looking to Apple for a more viable long-term alternative to help its publishers and advertisers".

Apple to the semi-rescue?

This reflects something we've been hearing from other companies, notably monetisation outfit Fiksu, which is suggesting that Apple will announce a first party cookie solution to the UDID problem as part of iOS 6.

"Here, we see a direct correlation between the money paid for an ad and the ability to track an ad," commented Jim Payne, MoPub's CEO.

"It's clear that Apple needs to address this issue with an appropriate alternative, because the damage to a publisher's bottom line will likely be material if UDID data actually disappears."

Indeed, this chimes with what he told PocketGamer.biz - "There is too much revenue at stake for Apple not to address the UDID gap".