I have a WP7 app that is connecting to my GAE backend via oauth. (Its probably not relevant that this is GAE, but I'm throwing that fact in anyway.)
I use the "btmpl=mobile" parameter to force a mobile UI in the Google auth pages, but I keep getting the non-mobile version which is really hard to decipher in a mobile browser. I extracted the Auth URI which looks something like:
http://www.myapp.com/_ah/OAuthAuthorizeToken?btmpl=mobile&oauth_token=randomlongstring
When I put that URL into desktop Chrome and Firefox and IE, I got the mobile login UI. When I put that URL into Android's stock browser, I got the mobile UI. But that exact URL when put into either a WP7 webbrowser control, or into the normal browser in WP7, produces the desktop UI. This probably has something to do with the ancient build of IE that WP7 is running, but really I have no idea what's going on.
This magically stopped happening with Mango.