"I believe eGroupware and Horde both include SyncML servers in PHP."
I think they just defer to funambol.
"There is also MultiSync in C which has several data engines, one that is SyncML"
Multisync became opensync, and is no longer developed as mulitsync, no? The SyncML piece of opensync does not really seem usable and the project seems to have been making steady but very slow progress for several years without a stable release.