Although we've been running iPhone OS 4 -- now known as iOS 4 -- since it was first announced in April, we actually haven't been able to try out any of the fancy new multitasking features, since no third party apps have been released with support yet. That's all about to change: as of today, Apple is accepting iOS 4 apps, which means we should see quite a few apps that take advantage of multitasking on the iPhone 3GS and iPhone 4, as well as make use of the 1500 other new APIs in the system when it launches on June 21.
That's definitely good news, but we've got a feeling Apple's unique take on iPhone multitasking will take a few people by surprise, as only certain tasks are allowed in the background -- and apps that need to run persistently, like IRC and IM clients, still won't really work at all. In fact, there's already some rumblings by devs that they need additional backgrounding tools in order to provide features that users are asking for; Instapaper's Marco Ament has proposed a network polling API that would allow his app to update content in the background, for example. We'll see how quickly Apple responds to these requests, but for right now we're just happy we'll finally be able to switch away from a Skype call or GPS app while using our iPhones.