As said above, most of the computing world celebrities we know of won't be recognisable to children but I've thought about this for a while and I've come up with the perfect answer.
Mark Zuckerberg http://en.wikipedia.org/wiki/M.....Zuckerberg
Recognisable to young people (by name at least) from the recent movie The Social Network. Recognisable by reputation because every kid spends a lot of time on facebook.
Inspirational because he was a young programmer who became very very rich (most kids aspire to fame and fortune). And relevant to our cause because he is a proper programmer, not just a business man.
Also from the Charity point of view Wikipedia says "Zuckerberg founded the Start-up: Education foundation. On September 22, 2010, it was reported that Zuckerberg had arranged to donate $100 million to Newark Public Schools, the public school system of Newark, New Jersey" which would suggest he could be very interested in the primary aim of the Raspberry Pi project.