From Michael Anissimov on the Singularity Institute blog:
Thanks to generous contributions by our donors, we are only $11,840 away from fulfilling our $100,000 goal for the 2010 Singularity Research Challenge. For every dollar you contribute to SIAI, another dollar is contributed by our matching donors, who have pledged to match all contributions made before February 28th up to $100,000. That means that this Sunday is your final chance to donate for maximum impact.
Funds from the challenge campaign will be used to support all SIAI activities: our core staff, the Singularity Summit, the Visiting Fellows program, and more. Donors can earmark their funds for specific grant proposals, many of which are targeted towards academic paper-writing, or just contribute to our general fund
A friend points out one possible way this reasoning doesn't work: charities can gain political power by quoting a larger number of individual donors. This would argue for giving $10 to several charities and the rest of the money to the best one.
I suspect that this reasoning is not only reasonably defensible, but also much more palatable; that the underlying biases are tested much less strongly by the policy conclusion "give the bulk of your money to one charity" than "give nothing to other charities". I will try to remember to use this version henceforth.