It is crazy to think that Latitude launched 2 years ago and took this long to get around to check ins. Google has always said that Latitude was supposed to tell people where you "recently were" not necessarily where you are at the exact moment.
Maybe two years ago we were not quite ready from a privacy standpoint to broadcast to the world where we are at the moment. But assuming that Google has had this functionality ready to go for 2 years it is crazy that they sat beside and watched Foursquare/Gowalla/Facebook dominate the check-in space.
With millions and millions of android phones now out there, this has the potential to seriously disrupt the above mentioned services.