preparing vs validating

[eluser]plainas[/eluser]
The validation class allows one to validate data using a customised callback function.
All the callback examples in the docs return either true or false.
It's also said that any native php function that accepts one argument can be used.

My question is:
Is it possible to use the callback trick to prepare data, or can it only be used for validation purposes?
Taking it a bit further, is it possible to use the same function to prepare and validate?