I posted this on Facebook in April 2012, not long after the 1940 Census was released. I rediscovered this post in November 2020 and figured I'd post it here since it's not doing any good there.
The 1940 Census is out at http://1940census.archives.gov, released (as by law) 72 years after it was taken. Unfortunately it's not indexed by name yet, so if you want to find your relatives you have to know their address on April 1, 1940. I've found both of my dad's parents so far, one in New York and one in Bennington VT (my grandfather was 28 and my grandmother was 24, and both were living at home with their parents until they got married just 4 months later - amusing in retrospect). There's a bit of a learning curve til you get used to how they organize their data.
Four easy steps on finding people:
1. Use Google Maps to pinpoint the address to a specific block between streets. I found that 1349 Grant Ave in the Bronx is between 169th and 170th. It'll also show you what the property looks like today.
2. Use the "search by location" radio button and then "browse" tab. Enter the state, county, city, and street via pull-down menu (sometimes you can enter a nearby cross-street). Using Grant Ave as an example, there are 17 possible Enumeration Districts (EDs) that Grant runs through, so now we have to narrow it down.
3. Use the Descriptions option to read the pages that tell which streets and blocks are in which ED. Click the "View Description" link to get to the pages. In my case, I found it on the third set, one of the pages says that ED 3-210B includes the block bounded by 169th, Grant, 170th, and Sheridan.
4. Then it's just a matter of going to that ED (click on the 3-210B link) and then go to the "Census Schedules" link for that ED. You've got to look at every page of listings (10-40 pages) until you find them. I found that it's easier to download the whole set rather than look using their viewer, but that's my preference.
Once you find them, there's some interesting data, but not a whole lot. You can find out who was living there, their ages, where they were born (state or foreign country), how much education each had, what their jobs were, how many hours a week they worked there, and how much money they made in the last year.