Google officials initially said that the Google Maps site wasn't working for Windows Phone users because the Internet Explorer versions that are part of Windows Phone don't use the Webkit rendering engine. This argument didn't hold water given that Internet Explorer on Windows 7, Windows 8 and Windows RT uses the same Microsoft "Trident" rendering engine as IE on Windows Phone does -- yet users on Windows were able to get to the Google Maps Web site with no problem.
I've seen a number of users claim that no one should want to use Google Maps on their Windows Phones, given Bing and Bing Maps are the preferred/integrated solution there. However, I can attest that Bing Maps are not always accurate when I've used them to search locally on my Windows Phone, so I, for one, wanted Google Maps access as a back-up.
I asked Google officials today if there was any more information about what led to restoration of Google Maps access and whether it may be discontinued in the future. I have not received any word back so far.
Update: A Google spokesperson confirmed the redirect is in the process of being removed for Windows Phone 7.5 and Windows Phone 8 users and is being rolled out worldwide in the coming days. Users who can't see Google Maps in the browser should make sure their phonese are set to mobile mode in IE. For desktop mode, the redirect removal is going to be coming soon, the spokesperson said. Google is not going to remove the redirect for Windows Phone 7.0 users, the spokesperson said.
Some Windows Phone users seem to be not-so-pleased with the way that Google Maps is working in Internet Explorer. Romit Mehta (@theromit), for one, who was having problems with type-ahead search and drop-downs.
I just tried it myself and got it to give me good enough walking directions for the search I attempted.
A couple of related things that might be worth pondering/reading:
Update: I am hearing from some Windows Phone 7.x and 8 users outside the U.S. that they still cannot access Google Maps. So maybe access is only restored (for now) for U.S. users? I will ask Google if/when I hear back as to what the rollout plan is. The worldwide removal of the redirect for Windows Phone 7.5/8 users is happening and will be done soon, the aforementioned spokesperson confirmed above.