The first major update for the Windows Phone 7 OS, known as Mango, has just been approved for RTM.  RTM stands for Release to Manufacturing, which means the base of the upcoming version of Microsoft’s mobile operating system is now complete.

But that doesn’t mean you’ll be seeing a Mango update for your Windows Phone any time soon; handset manufacturers like HTC and Samsung, along with carriers like AT&T, Verizon and T-Mobile in the United States, will now have to tailor the base OS for specific Windows Phone handsets, which can take some time to complete.  Also, carriers have an annoying amount of say-so when it comes to WP7 update releases, which could lead to further delays.

We have no timeframe for Mango’s release other than “this fall,” which could technically come as late as mid-December.  But it’s more likely that updates to Mango will come in October or November, though Microsoft is no stranger to product delays.

Windows Phone 7 “Mango” will include Internet Explorer 9 with HTML5 support, threaded email, text and IM, third-party app multitasking and more.  If you’re curious about Mango, head to Engadget for a detailed preview they posted in June.