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.