I recently tried to sign up a Windows Live ID with a non-Microsoft email. The signup page from live.com is here:
but the page doesn't allow you to use email other than @hotmail.com and @live.com. However when you place the cursor in the box and click the "Get help with this" context link on the right, you'd notice the instructions do say "–Or– Select Use your own e-mail address." Well, something isn't in sync here. Then, if you follow the "Windows Live ID sign up page" in the instructions, it actually brings you to an otherwise identical signup page, which does have the "Use your own email address" checkbox.
Per the spirit of a good conspiracy theory enthusiast, we'll have to rule out the chance of this being a mere overlook. :-) Apparently it's an concerted effort on steering to Microsoft services with perfect defendability on not acting like a monopoly. Well, to an honest consumer, it still feels like a trick. Either play like a whole-heartedly open-standard vendor, or be forthcoming about not being one. Either way I'd like the big M$ a little better.