An endorsement from Oprah is like the holy grail for an under-the-radar author, inventor or wanna-be lifestyle coach—but will it be a boon for Twitter, or will the inevitable surge of new users overload the service right when it gets the biggest break in its three-year history?

SEE ALSO: Twitter Added 5 Million Users In March

Twitter-fiend Ashton Kutcher will appear on the Oprah Winfrey Show tomorrow, talking about his race to the million-follower mark with CNN, and Oprah is scheduled to make her first tweet on-air. Given the tremendous influence Oprah has on her viewers (they practically bought out Amazon’s entire supply of Kindles late last year, after she endorsed the reader), Twitter is all but guaranteed to get thousands of new users tomorrow. But the startup has already been struggling to handle the constant flow of new users coming from its surge in popularity (see the Twitter Status blog, which records issues like disappearing tweets, people losing followers and missing avatars for the past few weeks). And those service outages were what had naysayers wondering whether Twitter would even make it through 2008.

Co-founder Evan Williams hinted at tomorrow being “a very big day” in a tweet earlier, so the engineering team is likely in crunch mode—but the service is already running slowly today. Coupled with the rash of account-jacking worms over the weekend, if Oprah’s endorsement and the ensuing traffic jam does cause Twitter’s service to grind to a halt—the downtime could send core users to rivals like Tumblr, someting Tippingpoint Labs’ Andrew Davis says is a credible risk.

Staci adds: A little after 2 a.m. eastern, CNN Breaking News sent out an alert that Kutcher broke through the million mark first. Best thing about it all: Kutcher’s $100,000 to Malaria No More. Maybe Oprah will match.