I try to use the multitasking function of iPhone 4. I want to run the application in the background and often send an HTTP request to send / receive data, is this possible?
You can request a runtime when your application is conceived.
See the Multitasking Developer's Guide
Although, depending on the intent of your HTTP requests, push notifications can be a viable alternative.
This may be necessary because you do not provide audio, voip or location services, so your application cannot run in the background for an indefinite period.
, "" iOS 4.x . , , / . , "", (*).
(*) . , " VoIP", , , -, (, " " ). , -, , .
" ", Undeadlegion, .
, iOS4 .
@Undeadlegion has suggested a plausible way to achieve your goal. To be more clear, take a look at my previous answer to a similar question in SO, iphone - Connecting to a server in the background