It seems it depends entirely upon your choice of test tool and experiment implementation methodology. With the exception of SiteSpect most CRO testing tools employ JavaScript in some means to render the test on the page which means that search engines robots and crawlers don't get to index the test content, they fall through into your default content. Apparently the good people at Google et al want their indexers to be part of the test.
From our own experience of conducting MVT and AB testing, which incidentally involves using a tool that employs the JavaScript method for content rendering, we've understandably had a degree of pressure from our SEO colleagues to minimize impact on their area during the course of the test. So we've arrived at a process that helps everyone, more or less. During the early part of our MVT tests we exclude all our search traffic, both paid and unpaid. We do this by looking for specific URL parameters that identify search traffic during the test page load event. If the visitor is classed as search traffic they fall through the test and are presented with the default (non-test) page content, as are the search engine indexing agents. We monitor the test variant performances and after a week or so (given enough traffic) we cull out the variants that perform in an adversely negative way (see early post regarding this culling procedure). Then, when we have a test situation that in theory is performing in a wholly positive way, when conversions are up over the original default, at that point we stop excluding the search traffic and bring them back into the test audience. This way we get more traffic volume to feed the test and the SEO benefits from an optimised content.
I think the concept we should always be focusing on overall in regards to SEO and CRO is that if during testing you can impact on SEO rankings in the short-term, in the long-term the site will benefit from the optimised content and an uplift in conversion overall.
UPDATE: 2nd Augst 2011 ~ Here's a useful statement from Google on the use of Google Optimizer and the affect on the SEO ranking
