As many people have reported, the tinderbox that was in charge of making the Windows nightly builds of both Sunbird and Lightning (named ‘solaria’) has gone Missing In Action, meaning that no nightly builds for Windows have been uploaded since 13 March. Unfortunately, all that we can say at this stage is that the build engineers are aware of the problem and are working as quickly as possible to rectify the situation. The timing of this problem could not have been worse, however, given that release cycles are currently in process or finishing for both Bon Echo Alpha 1 and Firefox/Thunderbird 1.0.x. Development is still continuing in spite of this obstacle, including moving towards Sunbird 0.3a2, so please be patient and nightly builds will hopefully be restored soon.
Update: A build made by ssitter is now online here. This is not a regular tinderbox build, so it is less official. But it is the best we can do at the moment. Happy testing!