Alphabet to Number and Number to Alphabet

Pure Bash, 51

Most of the rest of the answers use some sort of conditional. This one dispenses with conditionals entirely, and instead treats the input as a base-36 number which indexes into an appropriately constructed bash-brace-expansion array:

a=(_ {A..I} {1..26} {J..Z} {A..Z})
echo ${a[36#$1]}

Ideone.

Erlang, 26 bytes

f([X])->X-64;f(X)->[X+64].

One of the few times where Erlang's string behavior is useful.

Python 2, 38 bytes

lambda x:x>''and 64^ord(x)or chr(64^x)

Test it on Ideone.