I have tried, without success to share a keychain between two accounts. In Keychain Access -> Edit -> Keychain List there is a Shared checkbox next to each keychain. If I click on that, the keychain disappears from the list of Keychains that you see when you open Keychain Access (and if necessary, click on Show Keychains). The keychain does not show up on any other account, either. The keychain is still there in ~/Library/Keychains.
Seems pretty useless, as does Apple's article "Mac OS X 10.4 Help -- Administering keychains for multiple user" at <http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh1775.html>.
Some idiot in the security world recommended that we split our account into administrator and non-administrator, and normally log on only to the non-administrator account. This forces me to try to share files, folder, and keychains, so I can get any work done while logged on from the administrator account. Mostly, I don't think Unix, or Mac OS X, was designed to share things. (This is not a Mac versus PC statement. I only have one account on the PCs I'm forced to use at work -- it may be just as hard or harder on a PC.)
This security advice is stupid! Don't do it. Do everything on one account or you will waste hours and hours trying to bridge the gap between them.
But I would like to know how to share a keychain, since Apple's help files SAY you can. Only it does not work.