So what you are saying, is that the micro USB plug does not stay inside your phone once you have plugged it in? I have a Samsung Galaxy S, and the MicroUSB port for is actually VERY tight-fitting and I have never had it feel like it would fall out. I'm assuming you are pushing the plug in firmly and completely? If so, then try different plugs to see if the problem is the phone, or the cable. If it is the plug, then the phone's replacement should be covered (in my opinion.)
I also deleted the second thread for this topic in the forums, to make sure that all the suggestions/help stay in the same place, and make it easier to follow.