Well I for one consider donating to a non-profit scientific endeavour very much a charitable donation!!
However, the value of WUs bought at auction messes things up. As I've stated before, by paying any money for WU completion credit, especially at auction, establishes that this credit has inherent worth. As a result, this credit becomes a commodity, and therefor there is no charitable act because you're getting something (other than tax credit) back.
Sorry NWM, I don't mean to offend, but that precedent really skews all definitions. Ignoring that one precedent, I think the only thing that could be used towards tax credit is the "supplementary cost" of running Seti@Home, which by design is very little indeed.