There were a few interesting announcements at WWDC06, such as a sneak preview of Leopard or Mac OS X 10.5. But I'm wondering whether some of those features are slowly migrating into 10.4, so that when 10.5 gets here, it will be interoperable with the current crop of 10.4 systems.
For one thing, the new CalDAV protocol (there's a good article about how to get the iCal server running on 10.4 at AFP548) seems to be already supported by 10.4 iCal clients. I had a bunch of iCals that were stored on my own WebDAV server; since installing 10.4.7, they've been automatically migrated to caldav: as the procotol, rather than webdav: which they were before. I'm guessing that if people have .Mac accounts, the .Mac server has already been embellished with caldav: support, and thus the iCal clients could now be updated as well.
Secondly, the new support for Spotlight (being able to search servers) is a desirable feature, but not totally without similar behaviour for 10.4. For example, although the spotlight field only shows items on the local drives, if you open up Finder and do an advanced search, you can search all connected servers. (Granted, this is a bit of a pain, but it's possible to do at present.) Also note that the spotlight mechanism for reading and writing spotlight indexes to remote volumes has been upgraded since it first came out; when you write indexes from a specific computer, you get a host-specific file being written to in the spotlight search folder. That should at least make concurrent updates to the spotlight index somewhat easier to maintain. Mind you, it's good to see that they're finally adding basic boolean operations into the search criteria; let's hope that it makes it easier to do things with it.