All these APIs will let iPhone apps work like those for Windows and Mac; users can switch among them without fears about the apps resetting, losing their place, or turning off, as is the case in the current iPhone operating system. I see real potential for business-oriented client programs that can monitor the iPhone itself as well as its apps, such as for security issues or policy compliance.
Mobile management over the air
Large companies often (rightfully) complain that you can't manage iPhones easily, unlike BlackBerrys, which use the RIM BlackBerry Enterprise Server (BES), or devices running Windows Mobile or the old Palm OS, which are accessed via various mobile management tools. Sure, you can create configuration profiles using the free iPhone Configuration Utility, but you can't guarantee that employees install those profiles -- which do things like restrict app downloading, require complex passwords, and use security certificates -- unless you physically connect each device to a PC or Mac that has the utility and the proper configurations installed. The other options -- having users download them from Web sites or installing them from emails -- don't let you track that the profiles are actually in place, which is critical for many regulated companies.
There are a bunch of mobile management products out there that can verify whether policy configurations are installed on your iPhones, but they can't actually provision the configurations for you. With iPhone OS 4.0, that'll change -- and IT will get essentially the same management capabilities it relies upon for old-school devices like the BlackBerry.
iPhone OS 4.0 will support a new mobile device management service that vendors can use to install these configurations over the air, without user intervention. That's on par with the BlackBerry, Windows Mobile, and the old Palm OS, though it does mean you'll need a tool like Good Technology's Good for Enterprise, MobileIron's MobileIron, Sybase's Afaria, and Trust Digital's EMM -- just like you do with BlackBerry, Windows Mobile, and the old Palm OS. Chances are that if you've deployed Windows Mobile or Palm OS devices, you're already using such a product, so you're probably looking at upgrading what you have rather than starting from scratch. (If you're a BlackBerry-only shop, you're probably using BES for these capabilities; thus, you'll need to add a second tool for the iPhone, since BES only does BlackBerrys.)
Multiple-account mail, unified inbox, and attachment opening
iPhone OS 4.0 gives email a significant boost. First, you will be able to have multiple Exchange accounts active at the same time; you no longer will be restricted to one account, as is the case today. (Plus, Exchange compatibility is being extended to the new Exchange 2010 Server.) Second, you get a unified inbox, much like Apple's Mail program for Mac OS X. Along with that feature come new navigation capabilities to switch among mailboxes, as well as threaded discussions and -- a key improvement for many users -- the ability to open attachments in compatible apps directly from your email.
For me, the multiple-Exchange-account and attachment-opening capabilities solve huge frustrations with the iPhone. Maybe this summer I'll finally be able to open a Zipped file attachment on my iPod Touch, then view or edit its contents. Or I'll finally be able to open a calendar invitation sent to my personal (non-Exchange) email account and add the appointment to my calendar. (I do need to point out that these email improvements catch up to what several competitors -- notably BlackBerry, WebOS, and Google Android -- already offer.)