For new desktops, you might consider KDE tools. KPilot is working with Kontact under Mepis. It was quirky and a little confusing because kpilot has it's own little viewer that does not change along with Kontact, but eventually I was able to sync up my old handspring visor. I can't say if this will sync back with LDAP, but it's worth a try.
As John's pointed out, you might as well use Outlook for synching if you already own it and the XP boxes are not hopelessly malwared out yet. I have no idea how to sync cell phones. Let me know how things pan out.
