Well, I tried to pull exactly two April Fools jokes today, and was successful at both.
The first was with the lady friend, who is in Florida working her ass off doing some testing. Not realizing that she's probably not in the mood for joking around (or remembered it was April 1st), I sent her an e-mail with the subject "Boston.com: Youkilis out for the season" with the following link: http://www.boston.com/sports/baseball/redsox/articles/2009/04/01/youkilis_out_for_season
She apparently freaked out for a minute before realizing what day it was, and was none too thrilled. Success!
The second attempt was to my co-worker Guy. He sent me and my boss an e-mail asking about some error message he was having. It was actually because of a license problem, but I told him "Well, since you and Guy [pronounced 'Gee'; the other Guy is Belgian] are using it at the same time, and your names are spelled the same, it's having issues." Guy bought it for a second, then remembered. But I had him. Success!
Overall it was quite a productive day.