It appears the 1.5.4 release is buggy. I've tested both 1.5.4 and 1.5.4.1 QT wallets, both crash when issued RPC commands.
I'm guessing this is why the remaining mining pools, exchanges and explorer are all still using the previous 1.5.3 version.
Here are download links to the 1.5.3 client if you'd prefer or need to run it (i.e for solo mining).
Tested and confirmed. v1.5.3.0a-Beta works fine for RPC mining. v1.5.4.1 as available on the top posting crashes immediately. I would take the version offline and replace it with 1.5.3.0, a wallet that can't handle RPC is useless.