Users of Cydia, the popular program for installing and managing unauthorized apps on jailbroken iPhones, were greeted this morning with some astonishing news.

"OK, don't hate me," wrote Cydia's creator, Jay Freeman, in a message to users. "I totally sold Cydia to Microsoft, and am now working on a .NET to Objective-C connector with the eventual goal of porting Silverlight. Apparently, Microsoft has a booth here at CTIA, and Bill Gates decided to show up in order to make the deal himself: he feels they need this to compete with Apple's mobile strategy."

Welcome to April Fool's Day in the technology world.

Of course, the Cydia joke is far from the only prank that will fool people today. Google, for one is legendary for its April Fool's jokes, and it's living up to its reputation this year with the announcement of Gmail Autopilot for automatically replying to messages.

Amazon.com gets in on the act as well, announcing that it's rolling out a new web service called Floating Amazon Cloud Environment, or FACE. The new service uses "durable, unmanned helium-filled blimps" coated in solar cells which generate power for servers. The blimps communicate with the ground via WiMax and on-board lasers, which "double as a defense facility."

The Baltimore Sun reports librarians in southern California, fearful of the rise of electronic books, "staged a symbolic Kindle burning" in a Los Angeles park.

And, as noted by the Detroit Free Press, browser maker Opera is putting out a tool for browsing the web using facial gestures.

We'll update this post throughout the day with any other good jokes we see. Feel free to point out more in the comments.

Update, 1 p.m.: Microsoft's Live Search team has a good one: "We are finally unveiling our long-awaited, updated and ready-for-today’s-internet new brand: MSN Windows Live Search on kumo@microsoft.com. We’ve done lots of customer research on each component of the new brand name, so we’re sure you will like it."

Joe Tartakoff has more in a roundup over at paidcontent.org.

Update, 3:45 p.m.: Microsoft's intranet is also getting in on it.