I added my Yahoo! account but was unable to see my buddy list immediately afterwards. However, after killing all telepathy processes, then restarting, I was able to see my buddies.
Can you reproduce this behavior with running contact list and check that the account is really not added to the contact list (eg. switch from groups to accounts view, the second button on the toolbar)? Try removing your account and readding it.
3 months with no additional info. Closing.