It's a dig at Steve Wright.
Almost every letter and email that Steve Wright reads out seems to include things like 'Great show!", "It's wonderful to hear your programme", "You're the highlight of my day!", etc.
Sarah Kennedy's listeners noticed this, and there were many references to it in her programmes. So people started writing to her with messages like "Steve Wright-type greetings to you". This quickly got shortened to 'SW's to you", which both praises Sarah's show and takes a dig at Steve Wright simultaneously.
Chris