You can buy topical anaesthetic for mouth use from most pharmacies. Have a look at for example "oragel"

(http://www.orajel.com/)

There are "miracle berries" that make sour foods taste sweet.

It'd be fascinating to try something that took the sweet out of sodas. Someone posted a link to the OpenCola project in this thread. It's probably possible to make a batch with no, or much less, sugar than regular coke.