As we mentioned this morning, Twitter is holding an event at its headquarters this afternoon at 4 pm PT, but so far the company is keeping mum on why we're all attending. But attending we are and we have a few hypotheses as to what could be coming, from the long awaited Twitter Analytics, to the launch of Annotations to a true business offering.

Whatever it is, it's sure to be exciting (right?) and our own Mike Melanson will be there to let you know what happens, as it happens. Come back at 4pm PT/7pm ET for our live blog.

Our friend Robert Scoble plans to offer a live stream from the event.

4:01: The Egg awaits us and we're told "a couple minutes guys".

4:09: @Ev is offering up some stats. Twitter mobile users are up 250%, 370k new signups a day with 16% on mobile.

"The usage on mobile really underscores what we think Twitter is for people...a real-time information network."

4:15: "You don't have to tweet," says Williams, "any more than you have to make a webpage to use the Web." Twitter can be used for content consumption as well. And....Twitter runs into some technical difficulties with the slide projector.

4:17: A lot of people are tweeting, says @Ev - "we're continuing to see unbelievable growth". Says we're seeing up to 90 million tweets a day. 25% of tweets contain links.

@Ev says we're here to talk about Twitter.com - "It's how most people experience Twitter", even though its something the "tech set" misses. Twitter accounts for 78% of Twitter use, with 14% using the mobile website.

4:21: "What we're announcing today is the new Twitter.com" (Cue inspiring video).

4:25: Shows a two-column redesign of Twitter.com.

Some features:
-Click on a tweet on the left and get more context and information in the right column.
-Goodbye Hovercards? Click on a user and get full contextual information.
-Click on a tweet, see full embedded photos and videos

Ev says "We've done deals with 16 video and picture providers...YouTube, Ustream..."

4:30: "This is a complete revamp. This is a new architecture. The entire front-end is re-engineered."

Shows an endless scrolling left-column, meaning you can continually scroll through your stream of tweets.

The new Twitter.com will be launching today for a subset of users and will be launching worldwide. It will be an incrememntal roll-out.

Biz butts in - and says that "If the Ev of six months ago were standing here, he would be passed out on the floor" and we should be happy because he's acting so "leadership-y".

Ev says "It's all about speed, responsiveness and discovery."

4:35: Let the Q&A begin.

Q:How long will it take to launch?
A:"Stability is our number one priority...we're not on a firm time frame."

Q:Scoble asks if the list feature will change at all.
A:Lists are easier to access, navigate, as part of redesign. "Right now you have to click on a name and it takes you to a completely different view. Now...it integrates this into the workflows." List limitations are unchanged.

Q:Maggie Shields from the BBC - Did you test externally?
A:Yes. Starting as early as May, June, had weekly user tests. "We've been continuously testing until now."

Q:Ben Parr from Mashable - What influence did mobile apps have on this?
A:We launched iPad app 10 days ago. They were in parallel development, which is why they're similar looking. "We designed device specific experiences."

What about monetization? Ev says the new design will cause more engagement, with videos and other links directly embedded. The opportunities are tremendous compared to the current site, they say.

Is Twitter expecting this to increase use of Twitter.com? The answer is "Yes."

4:46: Twitter has updated its site to include a new product page.

Q: Will more pageviews kill Twitter?
A: @goldman says the redesign involves a new architecture design as well, so it will handle increased traffic. "We think this is both a product enhancement as well as a stability enhancement." It's a "significantly different" architecture.

Q: Are there new team features?
A: Not in this release.

Q: SearchEngineLand asks if there are any changes in search we'll see.
A: Search is much more integrated overall. It's a "onebox" - you'll see a search pane that has real-time results, but also top tweets and people that match, as well as other things.

"This does not take down Google," says @goldman. "We haven't talked to advertisers yet."

Twitter has introduced a new logo

4:55:That's all she wrote, folks. The Q&A is over and the new Twitter.com should be rolling out for select users.

Update: The video is now available: