The next iterative release of TweetDeck, which you can snag here, adds the ability to autocomplete Twitter user names any time you type "D" for a direct message, or an @ anywhere in the tweet.
Immediately, a small box will pop up and with the more letters you type, TweetDeck will display all that fit the string. It doesn't just look for first letters either, so if you type 'gray', you'll get @louisgray, not just all names that start with @gray, for instance.
See Iain's video below:
If you're sending a tweet to a friend whose name you type often, the autocomplete might actually slow you down, but if you're unsure on the spelling, or it's a long name, TweetDeck has found yet another way to enhance your Twitter experience. It will be included in the next full TweetDeck release if you're not the type to play with early builds.