I'm considering placing some adult xxx stub content and links on one of my unused domains. Depending on the outcome of the experiment, perhaps I would like to go back to non adult content in a few months for this domain. What can I expect in terms of google position after this move? Will I be penalized? If so, for how long? The targeted search phrase will be the same for both adult and non-adult version.
As John Conde noted, it's a long standing myth that Google penalizes a web site for having adult content. They don't. The myth came about because so many adult webmasters resorted to blackhat tactics to try and rank in a highly competitive niche. They routinely get their sites penalized or deindexed because of these tactics, not their content.
There is some potential harm that could hold over from this, beyond a pre-existing penalty. First of all, your domain may still rank for adult terms. Now if you want that traffic, that's fine, but if you don't, it can be a pain to deal with. Another part of this is that your site may be filtered out if someone is filtering out adult content on Google and your domain may still be filtered for a while until the new content is spidered full and old content is deindexed.
Also, your domain may be on an "adult content" list operated by "net nanny" programs and corporate firewall software. This, while not directly dealing with Google, can cause you problems depending on your target traffic.
Having adult content does not cause Google to penalize a website or domain as having porn on a website is not against their terms of service. If you change the content to be non-adult in nature Google will adjust the rankings for those pages as it crawls and discovers the new content. So making that change won't be an issue if you do decide to do something like that.