Looks like Friday is going to be the big day. From The New York Times:

Senator Hillary Rodham Clinton is moving to suspend her campaign and endorse Senator Barack Obama as the Democratic presidential nominee, probably on Friday, according to a senior adviser to Mrs. Clinton.

Her decision came after Democratic members of Congress urged her Wednesday to leave the race and allow the party to unite around Mr. Obama.

Mrs. Clinton is likely to make the announcement in New York City, an aide said, although no final venue has been chosen.

Her decision, first reported by ABC News on its Web site, came after a day of telephone conversations with supporters on Capitol Hill about what she should do now that Mr. Obama has claimed enough delegates to secure the nomination.

Mrs. Clinton had initially said she wanted to wait before making any decision, but her aides said that in conversations, some of her closest supporters said it was urgent that she step aside.

UPDATE: From an email Clinton sent her supporters Thursday morning:

I wanted you to be one of the first to know: on Saturday, I will hold an event in Washington D.C. to thank everyone who has supported my campaign. Over the course of the last 16 months, I have been privileged and touched to witness the incredible dedication and sacrifice of so many people working for our campaign. Every minute you put into helping us win, every dollar you gave to keep up the fight meant more to me than I can ever possibly tell you.

On Saturday, I will extend my congratulations to Senator Obama and my support for his candidacy. This has been a long and hard-fought campaign, but as I have always said, my differences with Senator Obama are small compared to the differences we have with Senator McCain and the Republicans.

I have said throughout the campaign that I would strongly support Senator Obama if he were the Democratic Party's nominee, and I intend to deliver on that promise.