Not all.

Some of those with both 2G and 3G infrastructure just use one card, the type they were using before 3G was around... like Vodafone.... whereas some, like Orange and O2 in the UK require prepaid users to get "3G compatible" SIM cards - I have never seen one but I presume these are USIMs. It's really dependent on the carrier's policies. Naturally, upgrading your SIM card to one that can be used on 3G (then again, you could question why the 2G cards can't use 3G... since Vodafone's ones can) means that you can always fall back to 2G if you go out of a 3G service area. I need to get my hands on these 3G SIM cards one day... and one day I will.......... Vodafone in Japan (now softbank) issued USIMs... I don't know if they are still USIMs these days.... their 3G is the same as ours, but before that, their 2G wasn't GSM, but rather some Japan-only standard. Compare that to NTT DoCoMo in Japan, they issue "UIM" cards (not to be confused with RUIM which are for CDMA phones) which seem to work in GSM handsets. Unfortunately I don't think there are many of us who have an in depth knowledge of SIM cards and how they work.... I'm certainly not experienced on the issue.

HTH, any more questions, feel free to ask!
Dan