USB synchronization security issues
What memory is offered on USB-Plug differs between devices. Most of them offer the external SD-Card (if present), some the internal eMMC card. Some even seem to offer a virtual CDRom drive (probably containing some device-specific stuff like drivers). But I've never heard of any giving access to the SIM this way, or even to system ressources. Of course, at least the latter is possible using 3rd party utilities (see Zuul's answer for an example). But access to the SIM should be almost impossible this way.

This all applies to Android up to Gingerbread. Starting with ICS (4.x), things become a bit different as UMS (USB mass storage) support was swapped in favour of MTP (Media transfer protocol). But basically, above said should still be true as to what is possible to access and what is not.