Bitcoin client which doesn't use wallet.dat?
This is not exactly what you're asking for, but might be helpful. Bitcoin v0.4 supports encrypting wallet.dat, so you cannot spend without entering the password. Basically you would do something like:


make a copy of wallet.dat
encrypt your copy with Bitcoin client v0.4 and set a long random password (one you will easily forget)


Voila, you have just created your read-only wallet. Because of 100 keypool you will even see some new transactions made with original unencrypted wallet.dat

You can download Bitcoin v0.4rc2 here.