Is there a name for replacing @ with "at" and . with "dot"?

As well as obfuscation, it is often referred to less formally as "munging".

Obfuscation?

Obfuscation is the concealment of intended meaning in communication, making communication confusing, intentionally ambiguous, and more difficult to interpret.

Edit: Of course, in this case we're talking about making the communication confusing for spambots, and not for humans.

Anti-Spambot Encoding ?

Of course the opposite then would be 'Anti-spambot Decoding' ;-)

But on a more serious note: I think @David is right with his 'munging'.
Even more specifically: Address munging would be the right term (according to Wikipedia).

I'd consider it a form of CAPTCHA. The Turing test being not so much the transformation "at"==@ and "dot"=dot, but finding and recognizing such disguised email addresses in a text that contains lots of other "at"s and perhaps even a few "dot"s.