Ok This is now the second time this has happened to me. After coming home from a two and a half month holiday i found that my mac would only boot into windows and after trying got fix permissions, reset PRAM and the rest i finally gave in and just restored my machine from a time machine backup its has been a month since then and my machine will no longer boot into mac! When i boot it displays a loading bar and that goes about 1/3 across before freezing for about 3 seconds then going to the normal loading screen with the apple and the grey beach ball. I then proceeded to choose my Macintosh HD at startup by holding alt and that failed too, I also noticed that the Macintosh HD read EFI Boot not Macintosh HD (note it was not in the progress of updating when this error occurred). The last time i booted into mac before this happened i was constantly getting errors from OS X stating that it couldn't backup to the time machine even though the disk was connected! After a while i stopped time machine and tried to run a backup myself as in right click "start backup now" or whatever it is that constantly was checking for changes and the next day i restarted my computer into Windows and it would no longer boot into mac. I am not worried about data loss but this has now happened twice and another user when it happened the first time stated he had been having this same problem and he had to reinstall every now and then! This isn't really acceptable to me and if this is a fault with the machine i am willing to have a major fight with apple (not court or anything but phone call full of yelling etc). This really is putting a dent in my full believing in Apple and i hope there is a fix for this, however considering it has happened under a fresh install i doubt its software related (i restored time machine but only restored data didn't format the entire disk, as in i reformatted it installed snow leopard then restored my files).