Keychains hold passwords, certificates and general secret stuff - only do this if you understand the Apple will have access to this.... well assuming they can crack your keychain password (which they probably can).

Synchronising a keychain across macs could be useful, for example, having a dedicated keychain for WiFi credentials.

I stumbled across this link (dated Nov 2011) and found that this still works!

The ~Library/Mobile Documents/ folder is pushed to all iCloud enabled computers, so I have created a new folder, and copied my WiFi keychain into it:

#!/bin/bash
$ mkdir ~/Library/Mobile\ Documents/com~linickx~icloud
$ cp ~/Library/Keychains/wifi.keychain ~/Library/Mobile\ Documents/com~linickx~icloud/

If you only have one keychain login.keychain, consider splitting out the really secret stuff and only sync'ing the stuff you want to share with apple.

Now open "Keychain Access", delete the original keychain and add/open the iCloud copy. On any other Mac, add/open the iCloud keychain. Once complete, any change to the keychain will be pushed to all Macs, simplifying password changes :)

I'm also using this to sync dotfiles!

You could use dropbox for this, one reason to use dropbox is that iCloud sync seems to be a bit hit n miss; however Dropbox already have enough of my secrets, I'm not suggesting that iCloud is more secure, it's just better to have many baskets.

FOOTNOTE: If your mobile documents folder isn't sync'ing, see this post by SteveX.