What version of OS X are you running? I had the same problem, but it worked after I upgraded to Snow Leopard.
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Starting DiabloMiner (GPU)...
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[4/25/12 4:32:52 PM] Started
[4/25/12 4:32:52 PM] Connecting to: http://api2.bitcoin.cz:8332/
[4/25/12 4:32:52 PM] Using Apple OpenCL 1.0 (Dec 23 2010 17:30:26)
[4/25/12 4:32:53 PM] Added Radeon HD 4870 (#1) (10 CU, local work size of 64)
[4/25/12 4:33:32 PM] ERROR: Invalid solution 1 from Radeon HD 4870 (#1), possible driver or hardware issue
[4/25/12 4:33:48 PM] api2.bitcoin.cz accepted block 1 from Radeon HD 4870 (#1)
[4/25/12 4:33:50 PM] api2.bitcoin.cz accepted block 2 from Radeon HD 4870 (#1)
[4/25/12 4:35:29 PM] api2.bitcoin.cz accepted block 3 from Radeon HD 4870 (#1)
[4/25/12 4:35:50 PM] api2.bitcoin.cz accepted block 4 from Radeon HD 4870 (#1)
[4/25/12 4:37:15 PM] api2.bitcoin.cz accepted block 5 from Radeon HD 4870 (#1)
[4/25/12 4:38:39 PM] ERROR: Invalid solution 2 from Radeon HD 4870 (#1), possible driver or hardware issue
mhash: 86.4/85.3 | accept: 5 | reject: 0 | hw error: 2