Yonkly is not a backup service but I just added a new feature that lets you import your twitter messages into your yonkly account, so try it out.
Login then go to your profile page at http://yonkly.com/profile and click the "import twitter messages" link half way down the page.
It is smart enough to not import the same message twice. Let me know how it works out for you and what you think about it.
This is completely off topic but I highly recommend reading "The Last Lecture". If you don’t know what it’s about watch this video (3+ million views).