It was probably from Gary Angel's blog, SEMANGEL, that I found out about SEM Cannibalization of Organic Search traffic last year (PPC Self Cannibalization) and I wrote about it shortly after starting Webmetricsguru.com in cannibalization by organic search results by PPC campaigns.

Gary's current article/post Measuring Search Engine Marketing (SEM) as a Channel goes beyond the other posts by putting forward that Marketing Channels can be cannibalizing each other; I find that a very interesting idea to explore.

www.Semphonic.com  has developed specific methodologies and experience to measure Cross Channel Cannibalization, though his focus in the Measuring Search Engine Marketing (SEM) as a Channel is on Search:

"…How do you measure organic cannibalization? It’s pretty trivial. The easiest way is to have a fairly extended dark period. Ideally, the dark period should be somewhat longer than the sweet spot for prospect conversion. You need to make sure you aren’t changing other campaigns during the dark period and you should also pick an historically flat time of year. You don’t want to pick March if you’re a tax preparation firm."

Seems to me that campaigns should be suspended periodically to how their presence/absence affects the rest of the marketing efforts (and that's usually judged by traffic and sales and where they come from).

"…Of great interest in the last year or so has been the interaction within a single channel. Specifically, how users use Search as a process. A number of studies have documented a general "tendency for searches to shift from sourcing on non-brand terms to brand terms as they move through the sales cycle. This can have a dramatic impact on your understanding of Ad Group and Search Effectiveness if you are using visit-based or "last-source" based campaign tracking.

"……These numbers can also be used to help understand what happens after a conversion or response. PPC Buyers tend, erroneously, to assume that everyone they source is a 1st Time Prospect. You may find, instead, that most of your PPC sourcing is existing customers looking for support or job-seekers looking for careers.

You might also find, for instance, that PPC prospects, once they convert, always visit your site via PPC. Depending on the circumstance, that may be a minor nuisance or a significant cost factor. If you have tight margins (as many publishers who arbitrage PPC do), then a visitor who always sources from Search is much less likely to be profitable than a visitor who only sources from Search initially.

This is not something I have had personal experience measuring, not in this way - but I think it would be good to do, from time to time.   The idea that all the channels might not be well aligned, while not expressly stated in Measuring Search Engine Marketing (SEM) as a Channel is suggested by the material.