Most applications should run as they did under the old OS, without any need for updates from their developers. But apps won't be able to take advantage of the new iPhone 3GS- or iPhone 4-specific multitasking features without their makers doing at least a little work and releasing new versions. That said, a small handful of apps seem to have trouble running on iOS 4, but updates are pouring into the store--chances are good that your app will either run fine as-is or get an update in short order.

As with previous major iOS releases, Apple has made the upgrade process fairly simple. Just plug your device into iTunes, and then sync it to create an up-to-date backup. (You can never be too safe--if iTunes doesn't back up your iPhone automatically, right-click on the phone in iTunes' sidebar and choose "Back Up.")

iTunes should notify you that a new OS version is available. If it doesn't, click the Check for Updates button on your device's Info page. A dialog box will tell you about the new version, and after clicking OK, iTunes will download and install iOS 4 automatically.