For an open source alternative (and my tool of choice) check out git-annex with s3[1]. joehy has a todo for git-annex and glacier[2] and someone has already submitted the beginings of a patch. When i first heard of Glacier I was excited about the possibilty of using it for a backend to git-annex, but then I read cpercival's discussion of glacier[3,4].
I've written glacier integration with git-annex and it's fully functional. Details in [2]. The Glacier retrieval pricing uncertainty just means that I store less in Glacier - but for critical data where the paid retrieval cost is worth it, I happily "git annex copy --to=glacier".
If I understand correctly, Icebox is not a direct competitor to Dropbox. If anything, it enhances the Dropbox experience, so you could argue that it could help Dropbox get more users/customers.
[1] http://git-annex.branchable.com/tips/using_Amazon_S3/
[2] http://git-annex.branchable.com/todo/special_remote_for_amaz...
[3] http://www.daemonology.net/blog/2012-09-04-why-tarsnap-doesn...
[4] http://www.daemonology.net/blog/2012-09-04-thoughts-on-glaci...
EDIT: Removed comment about competition.