I didnt search the bug trackers before posting here nor did I file a bug myself.
I discovered this behavior last night and was able to reproduce the break of sync by merging my phonebook with my IM (msn-pecan) contacts repeatedly. Currently I dont have anything merged and I'd like to observe how the sync behaves without me merging before I file a bug. If it will do well for the next couple days and breaks again once I merge, I can file it as a bug.
But maybe other users can reproduce my findings in the meantime while I wait for my unmerged sync to break again ;-)