Since Twitter gave up on providing IM and sms access to their service, leaving most people stuck with web and api access there's been a variety of apps surfacing. Personally I use TTYtter a Perl command line app, nothing pretty, but it works for me.
To do so you need to edit the .ttytterrc file in your home directory so that you replace http://twitter.com/ with http://identi.ca/api for any urls, and disable polling for direct messages as identi.ca does not support these yet. The changes are as below:
url=http://identi.ca/api/statuses/friends_timeline.json rurl=http://identi.ca/api/statuses/replies.json uurl=http://identi.ca/api/statuses/user_timeline wurl=http://identi.ca/api/users/show update=http://identi.ca/api/statuses/update.json dmurl=http://identi.ca/api/direct_messages.json frurl=http://identi.ca/api/friendships/exists.json dmpause=0