Each seed of 10.5 has seen the odd new little thing pop up, for example, there is a new look Network Preference Pane, nothing major by a long shot, but I think it's vast improvement as it makes it easier to see what is going on. There are no massive Top Secret features in the latest seed, and if there are then they aren't exactly obvious.
If there are doing to be new features and things that developers have to work with, then we are going to need time to get our products ready for 10.5, what's the point in releasing an OS if most of the developers have not had a chance to test, let alone have to re-code their applications to work under 10.5. Not all apps just need a simple recompile, some integrate at a very low level with the OS and could need a lot of recoding to get them to work, as was the case with going from 10.3 to 10.4, and to a somewhat similar extent from PPC to Intel (in the same was as it was from 68k to PPC all those years ago).
I'm not expecting to see 10.5 until mid to late June, which would tie in with the "Spring 2007" date that has been mentioned, and also tie in with the rumors about the date of WWDC. If I recally correctly, the end of spring is 21st June, so I'm expecting an anouncement about the Top Secret features and anything else to be between now and April. The ship date will still be mentioned as spring, and more than likley we'll see 10.5 released at the rumored mid june WWDC.