the bubbles form on any impurity in the glass. any scratches or dings will disrupt the stability of hte fluid and release the carbonation. so, sometimes, if you have a brand new glass and you put seltzer (or anything with carbonation) it wont be fizzy until it something disrupts the uniformity of the liquid. some really fancy restaurants scratch the bottom of champagne glasses on purpose so the bubbles appear in a certain pattern, and it looks awesome. ALSO, the same thing can happen with boiling water. if you microwave water in a new mug, it can super heat, go beyond the boiling point without bubbling, and then explode when you put a sugar cube or a spoon in it.
http://www.youtube.com/watch?v=ZAqqpDF4bVw