There is a similar "Gray hat" SEO strategy, which involves buying domain names that include keywords you want, and then 301-redirecting them to your site--without updating their whois information. I wouldn't do this kind of thing unless you can devote a lot of attention to it.

You can create content-light pages like the ones Patrick does, for, e.g., encryptedbackupsonline.com--each page on the site would have another title from the same keyword cluster, and each one would end with a call-to-action like "Tarsnap offers [encrypted backups online] for security-conscious power users." (Where the brackets indicate a hyperlink.

Bonus points for having a landing page on your own site that targets those keywords.

(As is probably clear from the above, I do SEO. Patrick's advice is extremely good, so if you have the time, you'll probably do best by just doing what he says. However, if you'd like to know what you might pay to outsource that process to a third party, well, my email address is in the profile.)

Do those shadow domains need to have anything link to them for them to be useful?

I think if you do a redirect there is no advantage to that. Travel to http://www.easterbingocards.com, there is content there for the search engine to index and return as a result (content that also of course has a lot of the keywords that are being targetted).

(Unless you're talking about doing a reverse proxy to the content of the tarsnap site, but I believe there is also not much advantage to that since the search engines pick up on duplicate content)

[EDIT: this is coming from things I've read not controlled experimentation. I don't think you can be an 'expert' without the latter so maybe I should not have responded]

Ok, that's what I thought -- I started doubting when I read Patrick's comments. Thanks!

The strategy that you have outlined has vanishingly little to recommend it. You lose the exact match domain bonus and split your link equity across your main site and the mini-sites.