Apple’s Tim Cook announced today that the iPhone 5 had racked up 5M unit sales so far, a number some found disappointing. But he also announced another impressive figure, one that developers should be very happy to hear: 100M iOS devices are already running iOS 6. Updated with stats from Pocket below.

This means that almost 25% of the 410M iOS devices ever sold are now running Apple’s latest mobile operating system, released just 5 days ago.

Some developers had been reporting 25%-35% as early as 48 hours after the release of iOS 6 last week. Some of those iOS developers, like David Smith, are now seeing as high as 45% and Marco Arment told me that Instapaper users are hitting the 49% mark over the last 7-day window.

Screen Shot 2012 09 24 at 12.14.11 PM 520x401 Great news for developers: Apple says almost 25% of all iOS devices ever sold are already on iOS 6

The discrepancy in the numbers has to do with the fact that I calculated the 25% based on all iOS devices, many of which are no longer in service or cannot run iOS 6. But, by all accounts, iOS 6 is being adopted much faster than iOS 5, which took 21 days to get to 21%.

This is huge for developers of iOS apps, because it means that they can move to supporting iOS 5 and iOS 6 only more quickly. Supporting older versions of iOS as time goes on becomes more difficult for a variety of reason. Among those is that the newer versions of iOS add support for great user features that are not available in older versions, but they also provide many backend conveniences for developers that allow them to improve and expand support for their apps more quickly.

The most attractive parts of iOS 6, which require building against the latest version of the SDK, seem to be Attributed Strings — which allows for more flexible rich text formatting without having to delve deeply into core text — and UICollectionView, which makes implementing the ever-popular ‘grid’ view incredibly easily in iOS.

Elements developer Justin Williams says that the quick adoption of the latest OS has him excited to mek”I was pretty quick to adopt iOS 5. I waited about 30 days and then made the jump. With iOS 6, I’m planning to move even faster given my who the market for Elements is: the early adopters.”

Arment also says that he is “considering requiring iOS 6 for Instapaper’s next major upgrade.”

People talk about the adoption rate of iOS a lot, especially contrasting it with Android, where Ice Cream Sandwich is still only at 20% after nearly a year. But it really is one of the major strengths of the platform. When developers can count on users upgrading fluidly and rapidly, they can plan to incorporate the latest features into their apps, free of worry that they’re alienating wide swaths of users stuck on older software.

As I mentioned in my iOS 6 review, this is Apple’s first major completely native over-the-air upgrade cycle. And — at least thus far — it looks like it’s paying off in spades.

Image Credit: Christof Stache/Getty

Update: The cross-platform read later service Pocket has posted some stats and they’re seeing massive adoption of iOS 6. They say that over the weekend they found that some 63% of users on Pocket running the latest version:

blog iOS 520x295 Great news for developers: Apple says almost 25% of all iOS devices ever sold are already on iOS 6

And some 20% of those were using the iPhone 5. I’d imagine the numbers are probably quite high for any popular app that updated its screen for the new iPhone as well.