Here's some code that returns a Contact object - reference Microsoft.Office.Workflow.Tasks - for the user who last modified a SPListItem.
public static Contact GetLastModifyingUser(SPWeb web, SPList list, int listItemId)
{
SPListItem taskItem = list.Items.GetItemById(listItemId);
return GetLastModifyingUser(web, list, taskItem);
}
public static Contact GetLastModifyingUser(SPWeb web, SPList list, SPListItem listItem)
{
string modifiedBy = listItem["Modified By"].ToString();
Contact modifyingUser = Contact.FromSharePointUserString(modifiedBy, web);
return modifyingUser;
}