Apple can take the position that any alteration to the shipped unit voids their warranty. The only exception has been typically for systems that allowed user memory alterations. However, if there was a problem, one had to retain the originally shipped Apple memory to verify the non-Apple installed memory was not the culprit. 😱
If you go to the iFixIt web site and see the details to disassemble the MBA, there are several opportunities to damage things. The hypothetical 128Gb SSD power use and heat release would need to be the same as or lower than the 80Gb Hd currently shipping. Also the thickness will have to be less than or equal to the 64 Gb SSD which implies the next generation of chips that will fit in the same form factor. 😱
If Apple installs a 128Gb SSD in the future, one could pay their inflated price and possibly have an Apple store install it and that would protect your warranty. 😉