I'm not such a fan of /tmp, it creates a fairly large security vulnerability; Imagine that you are a user on a solaris system where you know some users have homedirs set to /tmp. Now imagine that the server has been recently restarted and /tmp is pristine and empty and you are a somewhat mischevious sort.
Createing /tmp/.ssh won't get you very far as ssh perfoms a number of integrity checks to protect you from sneakyness here, but think about .Xauthority files for instance, I could create an xauth cookie that I know, then put an Xauthority file in /tmp and wait for a user to log in. They'd potentially 'reuse' our version of the cookie and allow us to gain control of their screen, keyboard and mouse. Alternativly, one could create a profile, .login, .cshrc, .bashrc or .kshrc that does a bunch of evil things as/to the user logging in.
Even worse/funnier they would be unable to remove or alter these files so they couldn't even fix it themselves if they noticed.

Why would sudo be affected by the homedir?