The Wikimedia Foundation, as a donor-driven organization, has a fiduciary duty to be responsible and prudent with its money. This has been interpreted to mean that we do not accept "artificial" currencies - that is, those not backed by the full faith and credit of an issuing government. We do, however, strive to provide as many methods of donating as possible and continue to monitor Bitcoin with interest and may revisit this position should circumstances change.
There was a breakthrough today, however, as the NYC Chapter of the Wikimedia Foundation has begun accepting Bitcoin donations. Whether this translates to an organization-wide policy change remains to be seen. Regardless, conversions such as these show that Bitcoin continues to gain broader acceptance as a real currency.
Bitcoins have been donated to the NYC Chapter of the Wikimedia Foundation, thus far (this is a live counter). Click here to get their donation address.
You can read more about this on the Bitcointalk Forums.