This quick post was inspired by a comment the previous blog entry about 3G security. As the comment mentioned, 3G security procedures are just used if the SIM card, which should actually be called a UICC (Universal Integrated Circuit Card) these days, contains a USIM (Universal Subscriber Identity Module), i.e. a folder branch and internal logic for 3G security. For details see here.
As also mentioned in the comment, many network operators allow the use of old 2G SIMs (i.e. UICCs with a GSM SIM folder) in their 3G networks. From the outside, a UICC with a 2G SIM and a UICC with a 3G USIM can't be told apart unless the operator has printed something on the SIM that hints its a 3G SIM card. In practice, it's even worse as many network operators still sell 2G UICCs today, probably because they are a couple of cents cheaper.
But this approach now backfires with LTE. Here, the 3GPP specification explicitly states that 2G UICCs can't be used. And indeed, when a user has a 2G SIM card (which he might just have bought recently) he won't be able to use LTE because either the mobile won't even try or because the network rejects the user. I've given it a try and it really doesn't work.
In other words, those network operators on the cheap side will have to exchange a lot of UICCs in the future when they go live with LTE and their customers with an LTE capable device will be stuck in 3G.