Happened to notice this when I visited the site in Linux:
The issue is with the font; Trebuchet is narrower than a lot of other sans-serif fonts, including all of the ones listed as fallbacks in the CSS (who exactly did you think would have Calibri but not Trebuchet?) and the one Linux uses as the sans-serif default. I'm not sure what you could do to ensure that the text always fits, given that much variation. I did notice that Droid Sans has roughly the same proportions; maybe you could set it up to use that as the standard fallback, embedded using @font-face.

The issue is with the font; Trebuchet is narrower than a lot of other sans-serif fonts, including all of the ones listed as fallbacks in the CSS (who exactly did you think would have Calibri but not Trebuchet?) and the one Linux uses as the sans-serif default. I'm not sure what you could do to ensure that the text always fits, given that much variation. I did notice that Droid Sans has roughly the same proportions; maybe you could set it up to use that as the standard fallback, embedded using @font-face.