Are you noticing a lot of data usage? I know your phone isn't unlocked, but if you see a lot of data usage you may want to try Push Doctor. My unlocked 3gs was constantly using data to download proper push certificates, and Push Doctor fixed the problem. I had absolutely horrendous battery life before trying this fix, and now my battery life is really good.
It may be worth a try.
@Kelly, following your suggestion, I added the NERV repo to my Cydia sources and downloaded Push Doctor but I keep getting a "Half-Installed Package" notice and can't use PD. I don't know how many times I had deleted PD plus all of its package components (one by one) and the repo. After that, I reinstall everything again, only to get the same message. Any idea?