Anyone know a good reason why not? This seems like a great answer to the issue.

(I suppose you may get web page layouts "jumping" if the downloaded font had very different spacing to the fallback font. EDIT: Ah, just learnt this is called FOUT. http://www.paulirish.com/2009/fighting-the-font-face-fout/ Still, I'd personally prefer FOUT to being unable to read the page.)