Soon I'll be buying a brand new Mac. I already have all the applications I want on the new one (or some of them, rather). However, I want to migrate my keychain.
How does one go about that? Is it possible?
Well, you should simply be able to copy your keychain(s) located in ˜/Library/Keychains to the new computer. I haven't tried this personally but according to this page it should work.
If your using MobileMe then the Keychain is synced automatically as part of the sync, or you can do a manual once of sync for all your dock settings, keychain information and applications settings.
I got MobileMe for a year free with my first Mac, however I am not sure if they still do it since the official crossover.
you can also copy the folder /Users//Library/Keychains, that folder should contain your keychains' information.
I have not tested it myself, though.
Following tested:
On Mac Os 10.6 Server, I couldn't move the old login.keychain in place of the current one: there is extra metadata on the login.keychain, which I guess the login keychain needs to be recognised. I'd be interested to see confirmation/refutation of this guess.