I suspect this is more about isolating the phone from the radio energy sinks of the human body (surface moisture on the skin & saline blood plasma) than anything to do with the glass itself.

Any suitably insulating (capacitive) enclosure (such as a "napkin swan", teacup, etc.) which preserves a largely vertical orientation (perpendicular to the local ground plane) should work just as well. We're still comfortably within the far-field http://en.wikipedia.org/wiki/Far-field performance envelope for transmission, but well within transmission-line power transfer regimes at these frequencies with a cell-phone-sized antenna.

If the glass were lead crystal (extremely unlikely in the bog-standard bar tumbler pictured), there's more potential for interference and a spatial locus of signal above the noise floor is possible. Typical window and utensil glassware is essentially transparent to these frequencies.