user323: When requesting getwork from bitcoind or a pool, the miner is expected to be incrementing the nonce value of the block, but can it also increment the timestamp value, or would such tempering cause a submitted work to be rejected by bitcoind or a given pool?

user610: Yes, the miner can increment the time stamp by a limited amount, but only if the pool advertises X-Roll-NTime as a feature. DiabloMiner and CGMiner both support this.
See the official spec for more information.

323: <p>When requesting getwork from bitcoind or a pool, the miner is expected to be incrementing the nonce value of the block, but can it also increment the timestamp value, or would such tempering cause a submitted work to be rejected by bitcoind or a given pool?</p>
 610: <p>Yes, the miner can increment the time stamp by a limited amount, but <em>only</em> if the pool advertises X-Roll-NTime as a feature. DiabloMiner and CGMiner both support this.</p>

<p>See the <a href="https://en.bitcoin.it/wiki/Getwork#rollntime" rel="nofollow">official spec</a> for more information.</p>
