I don't know if this is because the dongle and the Arduino are connected direct to one laptop.. When it tries to connect to a website, the LAN connection all fails.. It says no network access. sometimes, it needed about 30 seconds and gained network access as well as internet access again, but still the Ethernet shield returned 0 connection failed, or by Xively returned -1.
I tried multiple possible combinations, but no use 
I don't know where the problem is!!