I couldn't imagine how it would be deactivated, but I found out: if you and the user you're switching to have different screen resolutions, depths or refresh rates, your fast user switching transition will be like a resolution change -- fade out-blue / fade-in ... to get the rotating cube back, just make sure everyone has the same color depth, screen resolution and refresh rates.
[robg adds: I thought we'd run this one, but a quick search finds that not to be the case, so here it is...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040211230138228