My goodnes, that's a big argh for such a small scratch. Don't worry about it! There's really no need to take any action whatever. You have to treat a steel frame pretty spectaularly badly to get any rust worth worrying about.
My thoughts anyway.
You've not owned a Fat Chance then?
I'd do as P20.
Nope, just bought a holey Whiskeytown Racer though. So I know a little bit about pisspoor frame design leading to rust 'issues'. I should have said 'Silly sealed frame tube designs aside, you have to treat...'.
In this case though the frame has high quality paint, will be looked after and definitely not kept outdoors, so I'd reckon there's no danger of this scratch leading to any noticeable rust in the lifetime of the bike's owner. That said, as long as the area is extremely dry, I wouldn't imagine sealing it could hurt. On the other hand - who wants a bike that looks brand new, without any battle scars, after they've ridden it 10,000 miles? Not I.