There's a new development on the Amazon comments threads (so far in Grady Harp's section): now posters' comments are being deleted not as 'deleted by Amazon', but as 'deleted by author', that is by you.
That means someone's been deleting your messages on your behalf, which means they have access to your account, 'cause one would need to log in as you in order to delete your comments as you.
For an example please check out this thread (it's just one place; there may be others; look carefully). See if there are any comments showing up as deleted by you that you did not actually delete.
If you see anything suspicious, it would make sense to change your password and remove credit card data (if you have any). Well, do whatever you feel prudent on the assumption that your account have been broken into. I do not exclude the possiblity of this being an inside job, so do whatever's necessary in your opinion.
Of course, you should also notify Amazon about this. It looks like someone's helping Grady Harp for serious hacking into Amazon (or maybe it's an inside job, who knows). And btw, why am I not surprised? Oh well...