Ars Technica posts a somewhat speculative article on why they feel that Leopard will not be ready by April. This is based on developer sources indicating that Mac OS X betas tend to ramp up over 6 weeks prior to launch, and that current builds are not currently stable enough.
Based on their best guesses, they point to "June" as a possible timeframe.
At MacRumors, we're not big fans of speculation but historically, it is true that developer seeds become more frequent as the final release approaches.
What's also clear is that Apple has stated that there are "secret" features that have yet been revealed, and it seems unlikely that these "secret" features will be casually revealed in a developer seeding. Typically, Apple would showcase new features at some form of media event.
At this point, we only have two upcoming Apple media events on our calendars: NAB on April 15th, and WWDC 2007 in June. The NAB event will likely be centered around Final Cut Pro and WWDC 2007 will certainly be Mac OS X focused.
Apple has stated a number of times that Leopard's target release is "Spring 2007" which would place the release between now and June.