A couple of months ago, I wrote a blog post that we were starting to roll out DKIM signing for our consumer email accounts sending from @outlook.com. These are for accounts that have been migrated from the old Hotmail/outlook.com infrastructure and onto our new Exchange Online infrastructure. Not all accounts have been migrated yet, so if you are an @outlook.com user and don't see DKIM signatures on your messages, it's probably because your account hasn't been migrated yet. DKIM signatures are also sent to external recipients, that is, outlook.com-to-outlook.com messages are not signed (we have other ways of authenticating for this type of direct traffic).
We finished outlook.com last week.
Next up is signing DKIM for migrated @hotmail.com users. Hopefully that goes a little faster.