errrrr.......it is already working on 3GS without wifi. as long as you have internet connection, be it wifi or 3G.

the only solution is to get your push notification working. my gf's phone has exactly same problem as yours. push notification not working....dunno why also as i jailbreak it exactly the same as mine.

lucky her phone has 3.1.2 SHSH on Cydia so i might help her restore.

you can try installing push fix and see if it helps?

your battery life should be very bad like this right? correct me if i'm wrong.

anyways, this is how i fixed the push notification.

1. installed Push Doctor from sinfuliphone repo. this is better than Push Fix.
2. installed SBSettings Push Notification add-on.
3. respring phone.
4. disabled Push Notification, using SBSettings toggle.
5. startup Whatsapp and it will prompt you saying that you need push notification on,bla bla bla bla, click accept.

if push doctor manage to fix your push notification then most likely whatsapp will work as well.

give it atry