It happened again today when I tried to export data from product.*. The traceback is following.
I'll test the patch from Stefan.

Environment Information : 
System : Linux-2.6.32-30-generic-x86_64-with-Ubuntu-10.04-lucid
OS Name : posix
Distributor ID:	Ubuntu
Description:	Ubuntu 10.04.2 LTS
Release:	10.04
Codename:	lucid
Operating System Release : 2.6.32-30-generic
Operating System Version : #59-Ubuntu SMP Tue Mar 1 21:30:46 UTC 2011
Operating System Architecture : 64bit
Operating System Locale : en_US.UTF8
Python Version : 2.6.5
OpenERP-Client Version : 6.0.2
Last revision No. & ID :Bazaar Package not Found !Traceback (most recent call last):
  File "/home/dell/Documents/Projects/openerp-server-6.0.2/bin/netsvc.py", line 489, in dispatch
    result = ExportService.getService(service_name).dispatch(method, auth, params)
  File "/home/dell/Documents/Projects/openerp-server-6.0.2/bin/service/web_services.py", line 636, in dispatch
    res = fn(db, uid, *params)
  File "/home/dell/Documents/Projects/openerp-server-6.0.2/bin/service/web_services.py", line 663, in exp_execute
    return self._execute(db, uid, wiz_id, datas, action, context)
  File "/home/dell/Documents/Projects/openerp-server-6.0.2/bin/service/web_services.py", line 645, in _execute
    return wiz.execute(db, uid, self.wiz_datas[wiz_id], action, context)
  File "/home/dell/Documents/Projects/openerp-server-6.0.2/bin/wizard/__init__.py", line 177, in execute
    res = self.execute_cr(cr, uid, data, state, context)
  File "/home/dell/Documents/Projects/openerp-server-6.0.2/bin/wizard/__init__.py", line 153, in execute_cr
    return self.execute_cr(cr, uid, data, next_state, context)
  File "/home/dell/Documents/Projects/openerp-server-6.0.2/bin/wizard/__init__.py", line 72, in execute_cr
    action_res = action(self, cr, uid, data, context)
  File "/home/dell/Documents/Projects/openerp-server-6.0.x/bin/addons/base_module_record/wizard/base_module_record_data.py", line 121, in _create_xml
    res_xml = mod.generate_xml(cr, uid)
  File "/home/dell/Documents/Projects/openerp-server-6.0.x/bin/addons/base_module_record/base_module_record.py", line 472, in generate_xml
    res_list,noupdate = self._generate_object_xml(cr, uid, rec[1], rec[2], doc, rec[3])
  File "/home/dell/Documents/Projects/openerp-server-6.0.x/bin/addons/base_module_record/base_module_record.py", line 388, in _generate_object_xml
    record,noupdate = self._create_record(cr, uid, doc, rec[2], rec[5], id)
  File "/home/dell/Documents/Projects/openerp-server-6.0.x/bin/addons/base_module_record/base_module_record.py", line 194, in _create_record
    childrecord, update = self._create_record(cr, uid, doc, fields[key]['relation'],valitem[2], newid)
  File "/home/dell/Documents/Projects/openerp-server-6.0.x/bin/addons/base_module_record/base_module_record.py", line 194, in _create_record
    childrecord, update = self._create_record(cr, uid, doc, fields[key]['relation'],valitem[2], newid)
  File "/home/dell/Documents/Projects/openerp-server-6.0.x/bin/addons/base_module_record/base_module_record.py", line 215, in _create_record
    field.appendChild(doc.createTextNode(val))
  File "/usr/lib/python2.6/xml/dom/minidom.py", line 1611, in createTextNode
    raise TypeError, "node contents must be a string"
TypeError: node contents must be a string