Rumors are that one of the most widespread malware, Conficker (or Downadup) might strike on April 1st, 2009. I gave a short interview to the German TV station ARD that was partly broadcast as part of the ARD Mittagsmagazin (also broadcast on ZDF at the same time).
To me, there is no reason to wait for a specific date such as April 1st, for a malware to become active. This is a bit different if the malware itself spreads via email, such as Storm worm, and is completely based on social engineering. Furthermore, in my eyes, system administrators are fully aware of the danger that Conficker might pose, once its activated and thus look for it with special attention on April 1st. Any other date would then - from the attacker's point of view - make much more sense.