My little sister is heading to Bolivia for a month. I really want to find a way for her to use my old unlocked 8310 while she is there. While I know she can use it to make and receive calls using any SIM card that she can buy there, I was hoping to find a service that could also provide her with temporary email access while she is there. Any suggestions? I'd even be happy if we could just find a basic data plan (i.e., no BB push service) so that she could have a instant messenger program running, thereby making it easier to communicate with people back home. Any suggestions are greatly appreciated!

Even you get her using things from a generic data plan, it won't be cheap on local PAYG. Your best bet is to get her T-Mobile unlimited international BlackBerry for the data communications and a cheap phone for the local SIM card.