If you're snatching up an iPad this weekend, one of the first things you'll probably do is start loading it up with apps. While you'll have to start from scratch (even if you already have an iPhone or an iPod touch), when downloading some iPad apps, you'll have the option to download the iPhone/iPod touch version, too. To find these applications, look for the "+" icon next to the iTunes listing indicating it's a universal app. If you choose this, iTunes will download both versions when you purchase the app. Sadly, if you already have apps on your iPhone that are now upgraded to iPad apps, you'll have to download the full version.
After downloading a universal app, your iTunes library will contain both the iPad and iPhone versions, and you won't have to worry about choosing between the two when syncing; the correct app will sync with the attached device.