14 DAY TRIAL // It has been known for quite some time that Leopard will be quad universal, supporting both 3264-bit and 64-bit computing on PowerPC and Intel architecture. Last year at WWDC, Jobs announced that the 64-bit support will be across the board, not only in the Unix command line but also in Carbon and Cocoa. That was last year, and during this year's keynote, Jobs only mentioned Cocoa.
Carbon is the Application Programmer Interface (API) responsible for the transition from OS 9 to OS X, and while it is older than Cocoa, it is not quite outdated yet. In fact, Apple uses it for quite a few of its applications. The lack of 64-bit support for Carbon would mean that many developers will not be offering 64-bit computing in their applications for a long time. This is especially true of cross platform products that would have to loose all references to Carbon, a process that is unlikely. Even Cocoa applications call on Carbon for those things that cannot be done easily with Cocoa.
Both during the transition to Intel and now, Cocoa was the clear winner over Carbon in terms of bridging the gap, and it really looks like Apple wants people to write Cocoa applications, but that does not mean it will be abandoning Carbon any time soon. Still, it makes little sense for Apple to spend resources porting all of the old APIs to 64-bit, just for the same of maintaining backwards compatibility forever. It seems that some parts of Carbon will be available in 64-bit form, which would make sense since even Apple uses them for their applications, but the older parts will be left out.
This partial support would tie in with the omission during the keynote, as Carbon won't be fully supported as was stated last year, but it will be supported.