Just want to point out one minor nit to pick..
NEVER specify "Arial" or "Helvetica" as fonts and simply fall back to `sans-serif` On OSX you'll get Helvetica, on Windows you'll get Arial, which display better on their given platforms.. a Helvetica-like font is the default on pretty much every given platform for web browsers.
I would say, that if a fallback font is specified in the font-face rule, then that should be used while the web-font is downloaded.
Also, it would be nice, as you say, if a fallback would be used when a glyph isn't in the specified font (only for that glyph).