The New York Times has reported that last year Apple considering making a strategic investment in Twitter. Apple would have a few hundred million into the company. According to sources with The Wall Street Journal, there are no current formal investment or acquisition discussions between Apple and Twitter now though.
Apple has tightly integrated Twitter into their new operating system and within iTunes. About a year ago when Apple and Twitter were in investment discussions, Apple wanted to protect Twitter from being bought out by Google or another rival, which would stop Twitter from working with Apple.
At the time, Twitter did not really have to worry much about funding because they had over $600 million in funding in the bank and had some solid revenues.