Xcode is no magic bullet. It won't save you if you have written non-portable code. But Universal Binary can accommodate as many processor architectures as Apple needs and it's reasonable to assume that Xcode will continue to support Universal Binary.
I'm not familiar with Qt but libraries like Qt generally need to make compromises to abstract away some of the differences between the UIs they support. If you are developing to multiple UI platforms the trade-off may be acceptable.
But if you are writing a Mac OS X app for Mac OS X only I don't see value in writing to Qt over writing directly to Carbon/Cocoa.