I know about the LX-50, and I also know that Cobalt switched to PIIIs when they dropped the K-6s. But that doesn't mean it will always be that way...
Sun may be thinking long term strategy here. Once they release their 8 core Sparc chip, they might consider blades with Opterons on them. We do know that Alchemy is working on a MIPS64 chip, so Sun could then have a line of blades with all 64 bit computing.