user323: When I called:
print bitcoin.getmemorypool()
print bitcoin.getwork()

using Python JSON for a testnet BitcoinQT server, I got the following responces:
{'previousblockhash': '00000000032e361b246e96c4e594523b6ff42bc0527e560f203fb20a08a86185', 'transactions': ..., 'version': 1, 'coinbasevalue': 5000400000, 'time': 1329342730, 'bits': '1c2336a4'}
{... 'data': '0000000108a86185203fb20a527e560f6ff42bc0e594523b246e96c4032e361b000000004c50804820b143e1583c8c49dd43a6ddb8bc777c57c2da4a92b39ee0e709bf504f3c290b1c2336a400000000000000800000000000000000000000000000000000000000000000000000000000000000000000000000000080020000', ...}

Comparing the previous block hash from GetMemoryPool and one extracted from the block header from Getwork:
00000000 032e361b 246e96c4 e594523b 6ff42bc0 527e560f 203fb20a 08a86185
08a86185 203fb20a 527e560f 6ff42bc0 e594523b 246e96c4 032e361b 00000000

From what I can see, the data is reversed in chunks of 4 bytes. Why is such a procedure done, and what other fields from those two RPC calls undergo such an operation? 

user659: The value of previousblockhash in getmemorypool output is the block hash in big-endian order.
The value of data in getwork output is formatted as I described in this answer to your question about how hash1 is calculated.
This byte order reversal on 4 byte words is done by a function called FormatHashBuffers in main.cpp.  It's only done for the data and hash1 fields of the getwork output.  There's no comment explaining why it's done.

323: <p>When I called:</p>

<pre><code>print bitcoin.getmemorypool()
print bitcoin.getwork()
</code></pre>

<p>using Python JSON for a testnet BitcoinQT server, I got the following responces:</p>

<pre><code>{'previousblockhash': '00000000032e361b246e96c4e594523b6ff42bc0527e560f203fb20a08a86185', 'transactions': ..., 'version': 1, 'coinbasevalue': 5000400000, 'time': 1329342730, 'bits': '1c2336a4'}
{... 'data': '0000000108a86185203fb20a527e560f6ff42bc0e594523b246e96c4032e361b000000004c50804820b143e1583c8c49dd43a6ddb8bc777c57c2da4a92b39ee0e709bf504f3c290b1c2336a400000000000000800000000000000000000000000000000000000000000000000000000000000000000000000000000080020000', ...}
</code></pre>

<p>Comparing the previous block hash from GetMemoryPool and one extracted from the block header from Getwork:</p>

<pre><code>00000000 032e361b 246e96c4 e594523b 6ff42bc0 527e560f 203fb20a 08a86185
08a86185 203fb20a 527e560f 6ff42bc0 e594523b 246e96c4 032e361b 00000000
</code></pre>

<p>From what I can see, the data is reversed in chunks of 4 bytes. Why is such a procedure done, and what other fields from those two RPC calls undergo such an operation? </p>
 659: <p>The value of <code>previousblockhash</code> in <code>getmemorypool</code> output is the block hash in big-endian order.</p>

<p>The value of <code>data</code> in <code>getwork</code> output is formatted as I described in <a href="https://bitcoin.stackexchange.com/a/2907/659">this answer</a> to <a href="https://bitcoin.stackexchange.com/q/2906/659">your question</a> about how <code>hash1</code> is calculated.</p>

<p>This byte order reversal on 4 byte words is done by a function called <code>FormatHashBuffers</code> in <code>main.cpp</code>.  It's only done for the <code>data</code> and <code>hash1</code> fields of the <code>getwork</code> output.  There's no comment explaining why it's done.</p>
