I know that Sky serves up a million+ hits per day
but they use a fair bit of smart caching hardware. I'm guessing that whatever platform you pick, you'll need to be thinking about a lot of caching to achieve those kind of numbers.
The book starts from scratch, and has an "in the trenches" developer focusses, so I hope it would be perfect for you guys! We use a twitter clone as our sample app (hubbub), so it suits your outlook too!
As for the IDE front, I use Netbeans
6.7 for all my Grails stuff these days, which is free and open source. It's not quite as good as IntelliJ, but it's still fantastic for Grails development, and getting better all the time! The code completion in Netbeans is pretty good (with dynamic finder support in the latest edition), and it supports debugging/breakpoints
too which is really handy.
The Eclipse IDE support for Grails (free from SpringSource -- the commercial entity behind Grails) is expected later in the year, and is likely to be top notch. But for now Netbeans is the best free offering out there. Check it out!
All the best for your app!