I have been suffering from not being able to use Twitter in Adium 1.4b17 for many weeks now. I have been using Twitterific in the place of Adium but it is just not the same. The fix (which seems to work for me right now) is pretty simple. Please keep in mind that if it stops working again then you will have to repeat this process.

1. quit Adium
2. in finder use the "Go" menu in the menubar and select the "Go to Folder..." option from that menu.
copy: ~/Library/Application Support/Adium 2.0/Users/Default and press "Go".
3. open the file called "AccountPrefs.plist" that is in that folder. Now you will have to find your twitter account in this list. Keep looking until you find it.
4. Once you find it locate the value called "Followed Timeline Last ID". Next to that you will see a bunch of letters and numbers.
5. With that file still open, go to "twitter.com" and login. once you login your profile page will appear. In the bar on the right side of your timeline there will be a link called "RSS Feed". when you hover your cursor over the link you will see a link in the bottom bar of safari:



for example, my user ID is 15886726, you need to take your ID number and replace the value in the plist value "Followed Timeline Last ID" with this number.
6. Save the plist file and launch adium again. Now you should be able to see your timeline once again.