I use several SIM cards, depending on the time of the day. One has free calls during the night, one is very cheap for data calls, one is cheaper during the day... sometimes I get confused: which is the SIM card currently inserted on my phone??? I wrote SIMNotify to solve this problem.
SIMNotify allows you to save the Phone preferences, including the wallpaper, and attaches it to the IMSI number of you SIM card. You can attach different wallpapers for different SIM cards, or when the GSM is turned off. Click the "Enable" checkbox, and that's all: when you will turn on/off the GSM module, your saved preferences will be restored.