Daniel Jalkut put together a little app called "Usable Keychain Scripting" a little while ago that was designed (what else?) to make scripting the Keychain a little more usable -- the interface included in OS X is bad, to say the least. He was apparently hoping that it would be better in Leopard, but no dice -- we're all using the new version, and it's still a pain to access the Keychain with AppleScript.
So he's updated UKS to version 1.0b3. It's still a free download (and I'm guessing he still hasn't added the ability to set values of Keychain items, but only because he was justifiably worried that doing something wrong would have terrible consequences). If you've already been using it, you should be happy to see a new version, and if you haven't tried it yet, but do a lot of Keychain scripting, hopefully it'll make the interface a little less painful.