Since none of the suggestions in the other threads worked, I wonder whether one could sideload all the updates. (In my case, I can't even get the APKs to download so that I can install them manually--every time I try, I get an error message about the file's integrity.) I've tried commercial installers--they just don't work. Or what about a conflict--maybe between market and amazon, or market and appbrain?? Or just a conflict with another app? On the Pocket PC, it often happened that when you installed one app, another stopped working. I remember once it took about a week to track down the conflict, which was that I had changed the way the date and time looked in the task bar. (Who woulda thunk?)
Related question; if I delete all the data from the download manager, doesn't that give Market amnesia?