Call SQLCLR from trigger or polling?

You don't say what version of SQL Server you're using, but if it's 2005 Service Broker was made for this. In your trigger, put a (short) message on the queue. Have your service execute a WAITFOR on the queue which will then trigger a RECIEVE to grab the message.

Better than spinning cycles seeing if there's nothing to do.

I, personally, would never call a web service in a trigger. I would write a service that polls the table for new data and calls the web service asynchronously.

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