Information had previously leaked out that said IE9 would be coming to Windows Phone 7 in the "Mango" update later this year. But, new information has surfaced that says IE9 will hit WP7 well before this update. This news comes from the same source who leaked out information regarding the pre-NoDo update and is believed to be reliable.
WMpoweruser states that not only will it arrive early but that it will also support HTML5 and Silverlight, they state:
We have a hot tip from the same person who accurately foretold the pre-NoDo update, that IE9 may come to Windows Phone 7 well before the Mango update towards the end of the year, and will in fact support Silverlight in addition to HTML5.
Neowin had previously reported that the "Mango" update was due in the holiday season of 2011 and was supposed to contain the IE9 update according to Zdnet. While we do not know which source is actually correct, the new information does come from a source who already nailed one WP7 update.
Currently, Windows Phone users are using IE Mobile, or better known as IE7 Mobile, with some IE8 featured added into it. The addition of HTML 5 support and Silverlight to WP7 will make its browsing experience a top contender for best in class browser.