The main portal for Sky television has relaunched written in Grails. Sky, also known as British Sky Broadcasting or BSkyB, employs 17000 people in the UK and operates the largest satellite network in the UK.
The site receives a million+ hits per day and joins all the other subsites also written in Grails:
Update 07/10: Couple of new things have arison since this was posted. Firstly, Sky actually employ 17000 people not 11000. Seems the Google sample data I obtained was out of date.
Also Glenn Saqui has a nice write-up on the architecture of Sky.com on Marc's Blog. Altogether the 4 subsites listed above receive over 110 million hits per month and run on a cluster of web layer machines and 2 db machines. Checkout the aforementioned link for more info.