If you are switching from Blackberry to iPhone 3GS, you must be trying to import all your Blackberry contacts to iPhone 3GS. When I switched to other phones before I always copied all my contacts to SIM and import them to the new phone. Most of the time the format would be different for the phones so you will end up editing the contacts. But with Blackberry to iPhone 3GS the process was really simple and clean.
I downloaded Blackberry Desktop Manager on my Mac Book Pro. After installing the BDM I had to reboot my Mac. After Mac restarted I plugged in my Blackberry on USB and started BDM (Blackberry Desktop Manager). Synced the contact with my Mac Address Book. After it was done I plugged in my iPhone 3GS with Mac and synced it using iTunes.
In few minutes and with few easy steps I’ve all my contacts on iPhone 3GS from Blackberry. I didn’t try to do it on my PC but I believe the process will be close to Mac, if not similar.