I have an alias to a folder on another user's iDisk. When I opened it under Leopard it would automatically log me in to their iDisk and mount it, as I have the account information in my keychain.
However under Snow Leopard it instead brings up the dialog for a user/pass for the idisk.me.com domain. If use my user/pass it fails, if I use their user/pass it works.
My best guess is that since I have multiple keychain entries for the idisk.me.com domain, the alias isn't specific enough. But it did work under Leopard so I'm hoping there's a new way to make it work again under Snow Leopard.