You can use a standard GSM 2G SIM with a 3G WCDMA network and in that case only the network requires the phone to prove its identity. With a USIM, the network also has to prove its identity.
So you're not automatically protected when using 3G WCDMA network. You need to upgrade to a USIM.
Note: The above refers to 3GPP GSM/WCDMA technology. Not sure about IS-95 (Qualcomm's CDMA) and its multiple variants.
Authentication is performed by the network HLR (Home location register) which is independent of the radio technology used. The procedure/algorithms are different for 3G-capable UEs with USIM, but the HLR can accept 2G users as well.
Bottom line is that if your 'home carrier' (the one that produced the SIM) allows it, you can use your SIM in any 3G network that is part of the roaming agreement of that carrier.