It looks like the rumor mill is heating up again, because according to Tech Crunch, Apple may be looking into buying Twitter.
One source says, "Apple is in late stage negotiations to buy Twitter and is hoping to announce it at WWDC in June." However, we've heard nothing else so far. There were rumors that Google was also in talks to buy Twitter, however, the founders say they do not want to sell.
With Apple's WWDC coming up, it's possible we could hear something, but given that Twitter doesn't want to sell, it's more than likely the same outcome will happen with Google trying to buy Twitter.
Update: It looks like this rumor has been debunked, because according to Kara Swisher, both Twitter co-founders, Evan Williams and Biz Stone, are in New York today to attend the 2009 "Time 100″ dinner and haven't been in any serious talks with Apple at all. The only serious talks Twitter have in with is with Facebook, and that fell flat as well.