For every ECDSA signature (r,s), the signature (r, -s (mod N)) is a valid signature of the same message. Note that the new signature has the same size as the original, as opposite as the malleabillity of padding.
Now that this is well known, I have to point out the following:
If some subset of clients rebroadcast transactions while flipping the sign of s then the transactions have different ids (because currently the signatures are included when hashing to find the transaction ID) and there may be some problems if the flipped version makes it into the block instead of the vanilla version as I believe the originator wouldn't recognise the flipped transaction has spent his coins.
ByteCoin