Microsoft has launched a browser-based Windows Phone 7 simulator, seemingly tailored to let the users of iphone/Android experience the "Metro" UI. The simulator will sort of work from the Windows desktop browser as well -- but as it's tailored around swipe gestures and touch input, mouse input tends to choke it up.
The simulator only scratches the surface of what Windows Phone 7 can do; while you can tap into certain screens (the People hub, the Phone dialer) and kinda-sorta simulate what you'd do there, the vast majority just kind of sit there. You're really just walking through a series of animated screenshots.
You can find it here from your smartphone.