I am wondering if Stack Overflow is really getting a new CEO or it's a part of today's prank.
It's what you might call an unfortunate coincidence. When I learned of Joel's intention to step aside, I too wondered if it was an elaborate (and not terribly kind) prank on the company. But no. The blog post is entirely in earnest.
But this question does explain why various people asked in comments to the announcement whether the idea was from the new CEO. It was not unless that person has access to a time machine. I expect the search will take some time. While the hope is that it will be several months, I would not be surprised if we don't have the position filled next April 1st. (If we have someone to announce in March, I'd argue to hold off until after the month ends just to avoid confusion. ;)
No, that is not a prank. The blog was already posted on March 28th and nothing there mentions April 1st, not even in the form of "three more days and I will officially resign" or "at the beginning of the next month, we'll contact some headhunters". We have all reason to believe this is a serious announcement.