yes, but not all iphone 4 apps are available on the iphone 3gs/3g/2g becuase they are too high resolution apps, or need a front facing camera.
you need an ipod touch or iphone :)
You would need to have an iTunes account.
Yes, iPhone does have several apps that function as task organizers. They even offer reminder service.
No, not all applications are priced. Many useful apps are free.
You buy the iPhone app
apple create the apps on iPhone
no you cannot use ipod and iphone apps on itunes, but you can buy the apps on itunes and put them on your ipod and iphone
Although iPads can download apps made only for iPhone/iPod Touch, the iPhone can only download iPhone or iPhone/iPad apps. Apps made only for the iPad do not appear on the App Store. Luckily most of these apps have an iPhone counterpart such as Angry Birds (iPhone) and Angry Birds HD (iPad only).
The good news is that IPhone tracking is available and the ... you can look out for such as the GPS tracking ... iPhone Jailbreak News - iPhone Apps - iPhone ...
If you unlocked your iPhone it is easy to access apps store all you need is a account to download thinks from there,if you jail braked your iPhone you may download thinks through cydia only .
As long as your computer has itunes that is authorised on the same itunes account as your iphone, then it will automatically transfer all your iphone apps onto the computer