After I upgrading to emacs-23 quite a couple of months ago, all these functions called *-other-window split my screen horizontally instead of vertically, as they have done for the last 20 years or so.
Getting it split vertically was really a pain. Today I found, that I just needed to set split-width-threshold to nil (well, I have never dreamt of that thing so far …), best done through customization. I should have searched for that solution months ago …
Update 2013-02-21:
I had written this article quite a while ago, but still I had neither remembered this article nor the name of the variable to customize. Well, this ocurred to me (again…) after starting to use W32 emacs [Link]. I really experience getting split frames to windows horizontally as a severe pain.