As the Xserve approaches its end-of-life, Apple has eliminated the SSD build-to-order option. Instead, you'll have to choose either a 160GB, 1TB or 2TB serial ATA ADM drive at 7200-rpm to fill each of its three bays.
AppleInsider and 9to5 both note that SSD promotional copy was in place this morning, but it has since been removed from Apple's Xserve page. Apple announced the Xserve's eventual demise in November. Remaining stock will be sold until January 31, and machines still in operation will continue to be supported until they give up the ghost.
If you still want one of these machines, hop to it as the clock is ticking. In the meantime, Apple has published the transition document [PDF] for those moving to the Mac mini server or Mac Pro with Snow Leopard Server.