It annoys me because it creates too many "boy who cried wolf" situations. I don't care if it's April 1st - if you present me with a serious situation, I'm not going to automatically assume you're joking. I witnessed a really bad prank last year. One of my coworkers' boyfriends called her saying he was in the hospital, letting her get all worried and close to tears before revealing the joke. To me, that's not funny - it's mean-spirited.
Though in some ways it can be fun, depending on the joke. Silly website layouts are kind of fun, for example. And I'm eager to do a joke review for Eden. So I guess it depends on the nature of the prank.