Thanks to Thomas Benisch from Sun the working hours feature is back and will be shipped in the upcoming 0.5 release. This feature, which was probably the most highly request user interface enhancement, was removed when we moved to the new views back in December 2005, because the old implementation proved too hard to migrate to the new implementation then. Due to a huge feedback from our community we brought it back. So tomorrows builds (Lightning and Sunbird) should incorporate the new UI to set your working hours.
A huge thanks goes out to Thomas for implementing this and to dmose and lilmatt for reviewing the changes.