Here's a code snippet which uses string, integer, boolean, 64bit integer, floating point, datetime

props = {'hello':'world', 'foo':42, 'active':True, 'deceased':False, 'large':8555111000, 'floating':3.14, 'dob':datetime(2011, 12, 14)}
sent_msg = Message('message with properties', custom_properties=props)
self.sbs.send_queue_message(self.queue_name, sent_msg)

The ones causing problems right now are booleans and floating point, the others are working fine.

See the newly updated test_send_queue_message_with_custom_message_properties (the failing code is commented out)