This happened to me. I think there was some sort of error in one of the records on the SIM.
I was able to fix it by installing an alternate SIM manager (Dopod SIM Manager 1.4.36.365 by NiTroGen.cab from
http://forum.xda-developers.com/showthread.php?t=320526). Uninstalling it restored the stock SIM manager. This allowed me to edit the entries on my SIM, but I was unable to find the error. Deleting all the entries did allow me to run the stock SIM manager again.
Currently I use SK Tools to export my SIM contacts to a text file from which they can be restored in case of a disaster. I have not checked to see if it is able to access the card in the case of this type of error, since I didn't get it until after I'd fixed the problem and it hasn't recurred since.