I would be trivial to add a JSON-RPC command to do that. Just use the IsValidBitcoinAddress function.
For extra credit, give an Object as the return value:
{
'isvalid' : true,
'ismine' : true, # true if isvalid and you have the private key (can spend coins sent to this address)
'version' : 0, # ADDRESSVERSION
'address' : '1frt6531....' # might as well echo back the address...
}