I apologize for the (hopefully temporary) inconvenience this might be causing you. I have been reading up on possible causes of this, and to be honest, it's still quite uncertain why this is happening, and seemingly at random intervals. As of this writing, there are 3 active email subscriptions and another 11 awaiting verification. A few have been able to subscribe successfully, which makes it a very curious thing why the others are getting the '404'. From my own experience, I haven't had any problems registering numerous email addresses and verifying the ones that I own. I have tried it with two of my own addresses to test the subscription process and both times went through without a hitch. (I have been using Chrome and Safari browsers on Vista and OSX Lion, from both my home network and the school's wifi and lan connections.)
I will continue to troubleshoot this (and hope that Google, who quite recently acquired Feedburner, will get its act together and figure out this bug). For the meantime, could you please leave a comment after this post to let me know whether or not you experienced this issue. (Please let me know as well if you are receiving, or have received, updates via email whenever I post something new...like this message.)
-mistahTee