Tweet4me is a useful service if you want to schedule your tweets and want to use your favorite Twitter client to do so.
The service lets you send scheduled tweets using DM. you can set the time and the date to send the tweets.
testing scheduled tweet with tweet4.me
— Long Zheng (@longzheng) December 22, 2011
The messages are scheduled at Tweet4me servers from where they are broadcast through your account at scheduled timings.
The service is in beta and has some limitations. Check it out here.