It is highly unlikely Apple would make any changes prior to releasing the updated OS to the public, but not impossible - the Golden Master build of iOS 4.0 featured the Game Center app, while the public version released soon after did not.
As has been widely reported, iOS 4.2 is primarly aimed at bringing the iPad up to speed, features-wise, with the iPhone and iPod Touch families, adding a bevy of features including multitasking, printing and content streaming.
However, that doesn't mean other iDevices have missed out.
While iPad users have been calling for the ability to print from their ''magical'' tablet since day one, iPhone users - with the exception of the iPhone 3G - also gain the ability to shoot documents to a shared printer. Printing is available from Safari and the Photos app, but not from Notes, and uses a simple pop-over menu.
Once iOS 4.2 is installed, iTunes also prompts the user to enable AirPrint, and will warn you if it is enabled without any shared printers available. Apple has promised that AirPrint will require no configuration, drivers or additional software and I was able to successfully print to an older Canon printer connected to a Windows 7 laptop.
Lock Screen Bug
A widely reported bug which provided an easy way to bypass the security code on any iPhone appears to have been fixed - following the steps to reproduce the bug now drops the user back to the lock screen.
It has been reported that MobileMe setup on iDevices now asks for an Apple ID. MobileMe logins were previously seperate to Apple IDs.
In-Page Text Search
Under iOS 4.2, iPhone users can now search for words or phrases in webpages within Safari. In-page texts searches use the existing search box in the top-right of the screen, with in-page results appearing below the usual Bing, Google or Yahoo results. Users can then flick through matching words or phrases, which are highlighted yellow.
Font changing in Notes
Purely a cosmetic change, but it is now possible to choose between Chalkboard, Helvetica and the default Marker Felt fonts in the Notes app. Changing font affects all new and exisiting notes, and it isn't possible to use more than one font in a note.
As Daylight Savings came into effect several weeks ago in Australia, I was unable to test if the DST bug was present in iOS 4.2.
Apple has notified developers that they may begin building and submitting iOS 4.2 apps for review, using an updated version of the iOS Software Development Kit. The release of iOS 4.2 will mark the first time the iPhone, iPod touch, and iPad families have been able to run the same version of iOS.
As always with a major iOS update, iOS 4.2 will require an iTunes update to 10.1, a minor bump from the current version 10.0.1.
Update: A new volume bar has also been added to the multitasking menu on capable iDevices, accessed by swiping to the right past the iPod controls.