Conficker, the virus, has a not so funny joke to play on April Fools Day, it will call home from a possible 3 million machines and be primed for attack, or not. Chances are the creators are just re-establishing control of lost machines.
When Conficker goes hostile it can be used for spam and cyber attacks, like crippling entire parts of the internet or attacking large networks such as government ones or even mass scanning for security holes for example.
The notion in the tech worlds security gurus heads are it wont be too damaging, seeing as severe damage doesn't make many dollars. Unless that is, they get paid for their botnet to do just that.