0.3.20 will support listtransactions '*' for all transactions.
Transactions since <txid> .... sounds dangerous. Are you trying to mirror the transaction database that bitcoin keeps? If you are, are you sure you'll deal properly with (rare) block chain re-organizations where transactions that you thought had one confirmation turn out to be invalid in a new fork of the chain?
I can imagine subtle bugs-- you ask for transactions since <txid> but that txid turns out to be an orphan transaction (double-spend on the losing side of a block chain re-org) so bitcoin returns... what?
All non-orphan transactions with transaction time equal to or greater than a given timestamp might work-- but what is your use case?