When Twitter users go to post a new tweet, they have in the past used a button that says "update" (at least from Twitter.com...that varies when you get into all the different apps Twitter users use). Now, that button says "Tweet".
We're not sure exactly when the change was made, but it appears to have been very recently, as there is quite an outpour of Tweets from users mentioning that they are now seeing the "tweet" button. Perhaps this is Twitter way of asserting some kind of ownership of the word and the brand that it carries.
Twitter made "retweeting" an official feature last year, after the community-born practice grew to mainstream use.
Interestingly, Retweet.com just sold for $250,000 this week.
The tweet button isn't the only new thing going on with Twitter's website this week. Twitter is currently in the process of rolling out geolocation features on the site, after having had such capabilities available via the API since November.
Twitter CEO Evan Williams will be speaking at SXSW early next week (WebProNews will be there covering it). We may get more interesting news from the company there (some expect an ad platform to be announced).
Update: Looks like some have noticed this for at least a couple days.