Are there any client applications for Twitter (PC, phone, etc.) that keep a locally backed up record of one's tweets? Twitter could really be useful as an ongoing activity log, but there's a risk of losing tweets over time because it's not really meant to be an ongoing thing, more of a "here and now" thing.
You can add the people that you are following in the Google Reader and you'll have, besides the backup of all your favorite tweets, also a really powerful search ability. Maybe it's not saving them locally, but I think this is even better, than have them locally on your device.
Are you saying that you are afraid that Twitter will delete some of your tweets one day? VERY unlikely!
I've installed Tweet Nest on my personal webserver, and it has all my tweets backed up: http://pongsocket.com/tweetnest/
You can use TwInbox. It's a pluging for Microsoft Outlook. You can post tweets, RT and post DMs. The tweets on your timeline, your own tweets, DMs and mentions also get saved locally to your hard drive.
One useful tool for this is Twistory.
It's a web service through which you can export your Twitter backlog feed to the calendar app of your choice (e.g. Google Calendar, iCal, Outlook). You can only access your tweet history back to the point when you enabled Twistory to record it though.