I received an email last night from Google:
“Congratulations on your continuing success with Google AdSense.
To thank you for your custom and help you to maximise the earning potential of your site, we invite you to try advertising with Google AdWords with a £30 free voucher”
It looks like this voucher is for brand new customers only as it must be used within 14 days of creating your Adwords account and is targeted at Adsense users. Luckily it arrived just in time for me as I’m about to start an Adwords campaign for a client 🙂