So far, this year has been one of discontinuation of some of my favourite web tools, including Windows Live Mesh, Google Reader, and now Astrid. There may be more to come. I've been wary of cloud services for some time, but the usual response when I mention this to people is that "they wouldn't shut it down" or, for the slighly less optimistic, "there will always be some alternative". It's not completely true. There's no reason that someone must keep running a particular service, and there's not necessarily a reason for someone to replace a given service with a similar offering if it is ever shut down, nor any particular reason to believe that an alternative will be just as good.
When thinking or talking about this kind of issue, it is more useful to say "when such-and-such cloud service is shut down" rather than "if" because, eventually, every cloud service we know and love is going to go away. The only real question is whether that will happen while we still want to keep using it. At least part of the time, the answer will be "yes". Facebook, YouTube and GMail will, one day, all be gone completely. This will be devastating to all their users and will result in many petitions and pleas that fall on deaf ears. Planning for such an event means either ensuring that our own software doesn't depend exclusively on that service (being interchangeable with something else, for instance) or finding an alternative right now that cannot be taken away.
Mokalus of Borg
PS - If you really want to own all your computing, you can only use desktop software.
PPS - On the whole, it's not a feasible plan these days.