There’s a rumor floating around today courtesy of a post at Pocket-Lint that Microsoft is set to announce the general availability of  the “Mango” OS update for existing Windows Phones as well as new Windows Phones hitting the market.

Previously, you’d be lucky to get Microsoft to say anything beyond “available in the Fall”, but according to sources Pocket-lint says the announcement, due on September 1, is being made because they want to take advantage of the press around the IFA event, Europe’s largest consumer electronics show. It is during this show that handset makers like Samsung, ZTE, Acer, and LG are expected to announce new Windows Phones with Mango installed on them.

However, Tom Warren at Winrumors is suggesting that the best we can expect is for Microsoft to announce several new Windows Phone 7.5 handsets, but that actual availability of Mango for existing Windows Phones won’t happen until later. (Read: “Fall”).

WinRumors understands that Microsoft will announce several Windows Phone 7.5 devices on September 1 but that the update for existing devices will not be made available immediately. Microsoft is understood to be preparing a rolling update instead of a single “big bang” approach. Windows Phone developers will be able to submit 7.5 applications on August 22 and the software maker will release the Windows Phone Mango RC development tools on that day too. Microsoft will likely wait until its development tools are fully baked and OEMs/carriers sign off on Mango before pushing the update to existing devices.

And therein lies the whole problem – the carriers.

It is one thing for Microsoft, or handset makers, to say that Mango and new Mango powered Windows Phones are going to be available on a certain date, but it is another thing entirely for the carriers to actually implement the upgrade.

I am sure that there will be no problems when it comes to the carriers selling new Windows Phones with Mango running on them, but as we have seen in the past with the NoDo update, as well as Android fragmentation, the carriers have no problem delaying updates.

As much as I would really like to see Mango roll out on September 1, I think the chances of that actually happening are pretty slim. On the flip side, if the handset makers do announce new Windows Phones with Mango, the carriers will probably get them on the shelves in pretty short order.

However, for the rest of the existing Windows Phone users don’t count your mangoes until they have ripened because Microsoft and the handset makers no longer have any say in the matter; and that isn’t likely to change any time soon.

[Cross-posted at Winextra]