Determine if an integer is a palindrome in a given radix (base)
GolfScript, 10 characters
~base.-1%=

That is an easy one for GolfScript if we do it the straightforward way. The output is 0/1 for false/true.
~       # Take input and evaluate it (stack: num rdx)
base    # Fortunately the stack is in the correct order for
        # a base transformation (stack: b[])
.       # Duplicate top of stack (stack: b[] b[])
-1%     # Reverse array (stack: b[] brev[])
=       # Compare the elements