I’ve been using git in my new projects these days, just because it’s cool — and it’s FAST! We use unfuddle to host our projects but some of our clients prefer to have their own accounts. You can only associate one ssh key pair to each unfuddle account. So thats fine, you can setup multiple ssh key pairs, github has some good explanations.
Here: http://github.com/guides/multiple-github-accounts
Basically you tell your ssh to use different keypairs for particular hostnames in ~/.ssh/config like so:
Host rorcraft.unfuddle.com
User git
IdentityFile ~/.ssh/id_dsa
Host ankoder.unfuddle.com
User git
IdentityFile ~/.ssh/id_dsa_ankoder
NOTE: If you’re using passphrase and you’re on OSX Leopard or if you’re using ssh-agent. You have to kill your ssh-agent process first.
Leopard has keychain integration built-in for ssh key passphrase by using ssh-agent. This will stop your configs working. Either don’t use passphrase or don’t store it in keychains.
ps xa | grep ssh-agent | grep -v grep
1240 ?? S 0:02.36 /usr/bin/ssh-agent -l
Took me a bit of time of figure this out.. but I still love git.
Rex Chung 
Not sure I understand the comment about ssh-agent being killed. Does it have to be killed before you generate your key pair or what?
I use keychain and ssh config files together every day. What sort of problems did you encounter with the config file when using keychain to store the passphrase?