I have two questions about checkpoints:
q1) Which block has most recently been designated a checkpoint.
q2) Could any body kindly post a link to any web-page that explains how and when these checkpoint locations are designated?
(I can't seem to find anything authoritative looking on the bitcoin wiki, or over at stackexchange, or on this forum that deals with that second question.)
Different implementations pick different checkpoints (or none at all— they aren't strictly necessary— they just close a couple of DOS attacks and reduce the risk of attacks via network isolation).
In the reference client the checkpoints are in checkpoints.cpp. 0.5.2 (the most recent full release) has its highest at 140700. 0.6.0 will release with its highest checkpoint at 168000. (the network is currently at 170201, we'll probably be at >173500 by the time 0.6.0 is released). For the reference client new checkpoints are just introduced periodically by developers at whim but always thousands of blocks back so that they're never placed anywhere near competition between chains.