For myself, I greatly appreciate having a service like Mailgun available to Rackspace customers - does Linode (or another company) have a similar e-mail service?
If you're using it for outbound mail, you might look at using Mandrill. They're much bigger -- it's MailChimp's backend, used by 5M companies. That means mail is their primary business, their ISP relationships are solid and you can count on them to have 24/7 staff if anything does go wrong. They're also quite competitive: 12K free e-mails per month, then cheaper than both Mailgun and Sendgrid.
You don't need Rackspace to use mailgun, but Sendgrid also works nicely and is independent. Just like you don't need to use EC2 to use S3 (or Route 53, etc).
Why? Stock price seems to be completely independent from actual performance; and even then, This is just a consequence of speculation, not any actual change in anything. Basic financials look tip-top. OpenStack infrastructure means you're probably not locked in.
Seems like a lot of work for something that may, incredibly hypothetically, be a bad thing. Based on a single news item, you're betting on not only a one-sided partnership, but one that somehow destroys your ability to use the company, in a bet that only makes sense to hedge against now if somehow in the future any of the things that make it easy to step away from a hypothetically-future-toxic-Rackspace are no longer true.
You should also consider the growing number of platform (PaaS) providers that make it super easy to manage the infrastructure yourself, and save you lots of money on the way. I believe that this is where most hosting providers are heading. Gone are the days that we should be dealing with infrastructure at the element level. While this is the wrong forum to list who to look at, if you have a Ruby on Rails app, look at Heroku or Ninefold