Vessennes' Bitcoin Foundation recently sponsored a full node incentive program that pays full nodes registered with a Bitcoin address weekly if deemed eligible. The incentive program's goal is to estimate the size of the Bitcoin network by finding all reachable nodes running bitcoind version 0.8 or higher, likely in an attempt to lower the barrier necessary for a possible fork.
PseudoNode acts as a full node emulator. When information is requested of it such as blockheight, or transaction data, it relays that request to another full node it is connected to and responds to the request with that information. A peer index is used to determine full node eligibility in the incentive program uses: the current block height, a valid random block when requested, and valid IP addresses of other peers. The author, basil00, elaborated on PseudoNode accomplishing all of these things:
PseudoNode does all of those things.
It can figure out the current height because other nodes tell it (in the "version" message). It acts as a proxy server for "getdata" requests, so it will return a random blocks on request. It collects peer addresses (and return on request) just like a normal node.
In theory PseudoNode could fool the current rules of the incentives program without acting as a true full node.