The behavior appears to be that if a checkbox field is added to the specification for liveagent.prechat.findorcreate.map and liveagent.prechat.findorcreate.map.doCreate, the creation process fails and the desired record is not created. As soon as the checkbox field is removed from the form definition, the record creation process executes as expected.
While the reason for this behavior is unclear, at least there are two possible workarounds. The key to both workarounds is knowing that the String value of a marked checkbox is "on", and the String value of an unmarked checkbox is blank or null.
Workaround 1: Workflow rule
Assuming you've already created your Checkbox field, you can create a complementary Text field to hold the checkbox value submitted through the pre-chat form. For example, let's say your Checkbox field is named IsEmployed__c. You can create a Text field named IsEmployedText__c, and bind the pre-chat form field to IsEmployedText__c.
To close the loop, you'll create a workflow rule that basically sets IsEmployed__c to true when IsEmployedText__c equals "on", and another workflow rule to set IsEmployed__c to false otherwise.
Workaround 2: Formula field
Start with a Text field, and then simply create a Formula (Checkbox) field that uses NOT( ISBLANK( __ ) ) to produce the equivalent Boolean value.
For example, create a Text field named IsEmployedText__c, and bind the pre-chat form to this field. Within Salesforce, create a Formula (Checkbox) field named IsEmployed__c, and put int he formula NOT( ISBLANK( IsEmployedText__c ) ).
