This just might be a good sign for Microsoft- but bad at the same time.
It would seem that the demand for the developer tools for Windows Phone 7 Series released out at MIX10 in Las Vegas have created such a demand that the http://developer.windowsphone.com is down.
This would not be the first time demand has outweighed capability. Last year when Microsoft launched the public beta of Windows 7 the servers could not keep up either.
Hopefully they will get the site back up and running to meet the demand signal for these applications.
Update at 9:00PM EDT: Here is an alternate source for the Windows Phone 7 Series Developers Kit download. All 4 programs in one download/install file.