I recently was working on a issue where customer had recently installed Feb CU 2008 and his lists started crashing. As such the list contniue to work fine, but it didnt allow editting of items.

On further troubleshooting, I isolated the issue to list with custom fields. And further isolating it to custom fields which take in data from customer input via these custom fields. In short custom fields which have the PropertySchema Tags in *fldtypes.xml. On research I found that this was a bug introduced in Feb CU. And the good news is that a fix is out in the KB -- http://support.microsoft.com/kb/973410

On checking the source code, the code some hows bungs up when reading this tag. The fix has been made by installing the KB.