You have been stuck with version too old well do not worry much about that anymore.
You can now easily download the new whatsaap for iPhone 3gs and above since most 3gs and lower versions are now unable to connect on whatsapp.
This is mainly caused by old whatsapp version or the worst case, old iOS versions.
iOS 4.2 and below will no longer run the current whatsapp even when you have installed the new one.
Here is the trick
To be able to do so, all you need is to upgrade you iOS to a higher version but you may have problems of failing to unlock your sim card, especially if your phone is not factory unlocked, your local network may be unreachble by upgrading to the new iOS.
Cheat your iOS version with iTools and upgrade to iOS 4.2