1. Apple is allocating all of the new zippy new dual-core chips for the shiny desktop G5s, because they're in higher demand.
2. PowerMac G5s have a higher profit margin than the Xserve.
3. Apple's server applications, Workgroup Manager, Xserve RAID, Xsan, etc. aren't optimized to take advantage of the dual-core architecture and thus wouldn't benefit from it.
4. Apple is skipping the dual-core chips in favor of jumping straight into Intel chips in their servers.
I suspect that the actual reason for the lack of dual-core Xserves is more because chips are being allocated to the desktop Macs (The Power Mac Quad is still not shipping) than the software reason or that they're going to jump straight to Intel, but who knows.
So will the Xserve ever get a dual-core chip? I suspect that it will after Apple gets enough quantity of the chips to satisfy the demand for new Power Macs. I hope that Apple isn't spending a lot of R&D resources on the Intel Xserve, because while great, they can sell a whole lot more Intel PowerBooks.