Yes, this slot is for 3G connection via internal GPRS/EDGE/3G WWAN modem in Mini PCI Express card form. Read service manual for Mini 12 at Dell site and you will see a pictures of modem installation process. Anyway, to use a 3G on Mini 12 you should wait a special WWAN version of Mini 12 from Dell, or do it yourself: solder to the motherboard a SIM card holder and a missed WWAN Mini PCI Express connector, install a WWAN antenna and a WWAN modem card.
Here is the photo of Mini 12 internals on which you could see a place for WWAN modem card:
Dell Inspiron Mini 12, 1.6 GHz CPU, 1 GB RAM, 40 GB HDD, 3 cell battery, MS Windows Vista.
Want to install internal WWAN modem, replace HDD with SSD or CompactFlash.
P.S. English it is not my native language, so, please, excuse me for its poor quality.