Sincere apologies on my behalf, as well. :( We, as not only developers but also fellow gamers, know how painful it is to lose your scores, kills and characters (though DLC characters you can always download for free) - something you've worked very hard to get. The fault was in build finalization process. We implemented anti-cheating measures a couple of episodes back which rely on matching version numbers. One of the version numbers was not updated and it caused the game to completely reject new save games. It pains me how small an error can cause such wide-spread havoc!
I just now finished coding a save file loader which takes a different approach to anti-cheating to make sure this cannot happen again. Also, now whenever there is trouble loading a save file, the game will not overwrite your save unless you specifically instruct it to. This means if the game is ever unable to load the save file, you will not lose progress and any bugs causing the failure can be remedied in an update without a loss of data.
I'll move on to unlocking the lost characters based on OpenFeint achievements now. Sorry guys! We'll make this up to you with even better updates.