What would happen if the IRS stripped the Wikimedia Foundation of its 501(c)(3) status effective Mar. 1, 2010?

In addition to US contributions not being tax-deductible, any further contributions/donations would have to be treated as taxable income by the Wikimedia Foundation (WMF). They'd be allowed to keep donations in any event, and even if they didn't get any more donations whatsoever they could probably afford to operate for at least 4-5 years on the money they already have - though they'd probably cut back on paid Foundation staff, and possibly move to cheaper offices in a less expensive town (they're currently in San Francisco, probably the most expensive place to operate in the United States). They'd probably want to keep the site running, though nobody would know why any more than they know why they want to keep it running now.

The implications of this are interesting: If donations dried up severely due to their being non-deductible, it's possible Wikipedia would start running banner ads to make up the "shortfall." That could bring in even more money than the donations - possibly far more. But at least the WMF wouldn't be siphoning money off from other organizations that actually do charitable work.

Setting aside quibbles over the effective date of non-profit status termination, the Foundation would be able to keep donations made prior to the termination date as long as they can show that they followed the rules of the IRS. Keep in mind that moral ethics and legal ethics are different. Deceptive fund-raising practices are not reasons for donation refunds as long as the fund-raising followed the letter of the law.

There could be a marathon pledge drive prior to the termination date, but given how many people are deluded about Wikipedia, the donations might continue to come in afterwards even though the donors know they can't deduct them from their taxes.

Since the Foundation spends less than a million dollars on hosting and technical expenses, and assuming that the millions of dollars they collected last year haven't been blown on other things, Wikipedia could easily stay afloat for another decade, at least. However, they might put ads on the site, temporarily, while they appeal the "unfair" IRS ruling, until they can get reinstated as a 501(c)(3) organization. I think Jimbo Wales would want to keep Wikipedia up even if forced to put ads on it. It's possible his speaking engagements would dry up if Wikipedia went down.

March 1? What about April 1?

This act is highly unlikely. The WMF hasn't committed the acts cited under the statute for revocation.

If it was revoked, however, they would be allowed to keep current donations, but not be allowed to accept new donations.

For the more speculative parts of the question, contact the WMF.

your contributions would no longer be tax deductible after that date