I am having a very similar problem with the exact same computer setup.  (27" Imac with a Radeon HD 5750 it is currently mining between 70-72Mhash)

The odd thing is that ALL of mine are being rejected about 15 minutes after I get them.  I am not sure if I set this up correctly or if I am starting it up right. 

1) I open bitcoin-0.3.2.1
2) I start DiabloMiner
Server host name or IP address [mining.bitcoin.cz]:
Server port number [8332]:
Miner username [sbuXXX.workXX]:
Miner password [XXXXXX]:


Below is my bitcoin.conf file located in ~/.bitcoin
rpcuser=sbuXXX
rpcpassword=XXXXXX
gen=1
server=1

What am I doing incorrectly?  Thanks for the help!

-VONNY