Zeezer,
"Now I want to use the service bus relay with topics in WP8"
Not sure if you are conflating two separate issues...
We do not have an SDK for Service Bus Relay for the Windows Phone. However, if your service is using any of the *HttpRelayBinding's, you can use a regular HTTP client from the Windows Phone app to communicate with the Relay endpoint for your service. You will need to include the ACS token for authenticating to Service Bus Relay in a custom HTTP header called ServiceBusAuthorization.