I first heard about using individual student whiteboards about ten years go, from a fabulous online teacher listserve, 4-8 Connection. Prior to that, I had used small individual chalkboards (often available in toy stores, but also from school supply companies) and children wrote on their boards with chalk and held them up. We practiced spelling, history, punctuation, science, math and a number of other skills in a whole-class setting. This was a very challenging urban middle school (I taught seventh grade at the time). Chalkboards were expensive, messy and a problem for students with asthma, however.
From the 4-8 connection group I learned you could get your own individual whiteboards by going to Home Depot and asking them to cut showerboard to your specs (they would often do the cutting for free); I got a class set of 10x10 boards and prepared them (for easier erasability) with a coating of Turtle Wax. Kids brought a (clean) old sock to keep in their desk and use as an eraser. Now that many dollar store chains carry cheap whiteboards, the Home Depot idea may not be as valuable but it is still an option.
Even though all our classrooms still have large chalkboards, there isn't room at those boards for all students to write simultaneously, and the advantage of individual boards (laminated light-coloured cover paper will also work well) is that using individual response methods increases engagement and academic learning time. A practical book for teachers who want some ideas on how to use individual response systems in whole-class teaching is a book from Sopris West entitled Think,Get Ready, Respond! which outlines not only the use of whiteboards but also variants like colored cards, Yes/No signs and so forth.
Using something students can hold up, rather than something on their desk or to be corrected later, has several positive effects. Kids who are anxious, inattentive, or low-achieving for a variety of reasons participate much more readily and unison responding, whether oral or visual-motor, allows the teacher to monitor how well the students are learning or applying the concepts and material taught. If there are a number of incorrects, the teacher can (without singling anyone out), back up and re-teach an earlier step. Or, s/he can present a "differentiated" task, e.g. a challenge task vs. a "regular" one and let students select which one to respond to. It provides a lot of useful information to the teacher on what students know and can do. S/he can mentally note which students need additional practice or a comprehensive review and address those students' needs separately at another time (thus not singling them out in front of peers).
Unison responding and group response systems have been validated as effective practices for a long time (DI of course incorporates oral unison responding in all programs). Some references I found are here:
JABA study on effectiveness of response cards
Meta-analysis of research on use of response cards
More practical information here:
Group Response Techniques (pdf file)
Using Guided Notes, Choral Responding, and Response Cards to Increase Student Performance (not sure what the URL is for this one - CJ)
Think, Get Ready, Respond! book I recommended earlier
Note to Paul B: Robert Marzano recommends the use of whiteboards and response ards, so you can safely come out of the closet. All the district pooh-bahs pay lip service to Marzano's principles even if they have never read any of his books (often the case).
If they don't want to read the books, some basics are here:
Conversation with Robert Marzano (pdf file)