From what I understand, so many people have Calibri on their computer anyway that it’s safe enough just to include it in the normal font list, followed by a list of similar fonts as a fallback. I don’t think it’s worth including it with @font-face, TBH.
But if your client insists that everyone get the font, the best thing to do is pick a font that looks a lot like Calibri that is free. Fontsquirrel has some nice ones, such as:
I am actually building a site at the moment using Calibri, and to be honest, it looks just the same on iPhone/iPad. I’m not sure if that’s because those devices have Calibri installed, or because the fallbacks I have look similar enough that there’s not a clear difference. This is the stack I’m using:
font-family: Calibri,"Gill Sans","Gill Sans MT","Myriad Pro",Myriad,Frutiger,"Frutiger Linotype",Univers,"DejaVu Sans Condensed","Liberation Sans","Nimbus Sans L",Tahoma,Geneva,"Helvetica Neue",Helvetica,Arial,sans-serif;
I’m pretty sure it’s only those who have Windows 7. So if someone has a Mac, any Linux distro, or Windows XP / Vista, chances are he won’t have the font.