Although OS 4.0 is available for everyone in the summer, not every feature will work with every model. Specifically background applications.
If you have the 3GS or the 3rd generation iPod Touch, all the features of OS 4.0 will work, including multitasking. If you have an iPhone 3G or the 2nd gen iPod Touch, you can install OS 4.0 but you won’t get multitasking.
As for those you you who own the original iDevices (2G iPhone or 1st gen iPod Touch), there was no mention of OS 4 compatibility for these models. Although when questioned, Steve Jobs said this:
The old devices will get the update, but they’ll miss some of these features like multitasking. If that’s an incentive for them to upgrade to a new device…terrific.
So to sum up: Get used to it or buy a new device. Will you upgrade just for multitasking? Or will you stick with 3.0? Let us know in the comments.