When the 1940 US Census becomes available to the public on April 2, 2012, images will be free on 1940census.archives.gov (until the end of 2013), thanks to a partnership between Archives.com and the National Archives. This means that anyone will be able to freely browse the images, but there will be no index so you'll have to know where your family members lived in order to find them. Volunteers are being recruited to help index the census through an indexing initiative called the 1940 US Census Community Project. You can sign up at 1940 US Census Community Project.
The 1940 US Census Community Project is co-sponsored by Archives.com, FamilySearch.org, and findmypast.com. The goal is to get the 1940 US Census indexed within six months, but if those interested can take a little time and index a few pages, perhaps it can be indexed even sooner!
If you have never done indexing before, you are encouraged to download the software at FamilySearch.org and help out with some of their other ongoing indexing projects. (If you have a FamilySearch ID, you can log in with that username and password). There is a short instructional video, then you can start indexing. There are different difficulty levels and most "batches" usually don't take much more than 20-30 minutes to complete.
I've recently started indexing at FamilySearch and now that I've indexed a few batches, my advice is to read the instructions that are always in a box in the lower left hand side of the screen, and take advantage of the "Lookup" feature, found under Edit > Lookup, where you can help decipher a person or place name for which you can read most but not all of the letters.
I hope many genealogists will volunteer some time to index at least a few images for the 1940 US Census.