Apple restricts 3rd party developers with lack of multi-tasking on the iPhone
We know that the iPhone can obviously multi-task now with the Apple applications, and even with jailbreaked apps, since you can perform tasks such as initiating an email download and then bounce over and surf the web and then bounce over and check stocks. Buried in the 100-page iPhone Human Interface Guidelines of the SDK documentation is the following statement:
Only one iPhone application can run at a time, and third-party applications never run in the background. This means that when users switch to another application, answer the phone, or check their email, the application they were using quits. (p. 16)
Wow, there goes all the plans for those great Instant Messaging applications that would provide you with notification and keep you logged in while you use your device with other applications. Something like an IM application would work if you only used Apple's applications in conjunction with it, but with what I imagine will be a large selection of 3rd party applications there would be no way to manage how people use their devices.
One of the main reasons I rarely use Palm OS devices is their lack of true multi-tasking. There are some Palm applications that developers wrote to multi-task as best as they can, but you can't surf the web, leave the browser and go do something else and then come back and expect to be just where you were in the browser again. Windows Mobile and S60 have multi-tasking support on their devices and these operating systems manage running applications pretty well. Based on how the iPhone has performed for me over the last year, I think Apple does an even better job at managing their multi-tasking in iPhone native applications and it is a shame that 3rd party developers will be limited with their applications.
I think there will still be some great applications, like games, SlingPlayer, and VoIP, that won't be directly affected by lack of multi-tasking, but there will also be quite a few that may never be developed because of this limitation.