As of doing a clean install of Mountain Lion, I can only access my Time Machine backups dating back to my upgrade (only a few days ago). My backups dating all the way back to March of this year are inaccessible, even though the dates for those earlier backups still appear in Time Machine and it shows that my 1 TB external HDD still only has 600 GB or so of free storage. So it doesn't seem that the backups were deleted, but at the same time I can't get to them. =/