I just took a look at the get_transaction_hash function in the Monero source code, and if I understand it correctly, the way the transaction hash is computed will change once ringct is activated.
The comments in the code indicate:v1 transactions hash the entire blob v2 transactions hash different parts together, then hash the set of those hashes
The 3 parts of v2 transactions are:prefix base rct prunable rct
Hashing the hashes of these 3 parts together gives the final transaction hash.
I suppose pruning the signature from the block chain will be possible (like in Boolberry), because keeping the hashes of the signature parts instead of the whole signature will still allow computing the transaction hash.