How does the rate of getwork requests correlate with the hashrate of a miner? In a setup where a standard bitcoind serves full difficulty targets to some mining nodes, how can I calculate the hashrate of a mining node from looking at the rate of getwork requests? Does this depend on the used mining program or the used mining extensions?
The standard getwork protocol forces the miner to send a new request every second. This means that if a miner takes longer than 1 second to check all nonces in a given getwork (2^32), it will be making a new request before it checks all values in a given request, thus giving you 1 getwork request per second regardless of the hashrate.

If the miner can solve the getwork faster than that, it would be making requests more often. In theory, you could use this formula to calculate the hashrate:

Hashrate = noncemax/time

Where time is the interval between the getwork requests and noncemax is the size of nonce field (2^32). However, this assumes no time delay due to the network lag and so forth, so it's an ideal scenario. Moreover, some miners discard unfinished work after submitting a share, so it would appear they are doing more work in your configuration than they actually are.

If you intend on using getwork extensions, specifically noncerange and rollntime, you could calculate the hashrate more accurately. Noncerange would change your noncemax to whatever size you set, and rollntime would allow you to measure time intervals greater than one second. However, not all miners support the extensions.