We’ll be integrating DKIM at some point in the not distant future. But, as you’ve noted, it’s a non-trivial bit of configuration and the dependency chain to achieve it is longer than many tasks.
You have two options for implementing it with Postfix: dkim-proxy and dkim-milter. I don’t have any knowledge of which is preferable, unfortunately, or whether they both cover outgoing as well as incoming (I’m pretty sure dkim-proxy does both). So, that ought to at least get you on the right track for the package you need.
I’ll try to figure them out in the next week or two, so I can drop a package into the repositories for CentOS.
As for Yahoo, as long as you don’t have other spammy characteristics, mail should still deliver to Yahoo successfully. I’ve tested as recently as a month ago, and it worked fine from our box (which is not graced with dkim).