Not via MMS or email unless you compress the file to the max, and even then it's highly doubtful that you'll be able to send the file through email. A two and a half minute video taken with the iPhone 4 would be 100Mb's plus. You're best bet is to upload the video and send the link to whoever wants it.
It should stream to their iPhone from the dropbox website as if they clicked a YouTube link. I ran into one issue where the recording it took on am iPhone 4 wouldn't play from dropbox on a 3GS. I assume that the 3GS couldn't play 720p video, because the video played fine on other iPhone 4's.