[The following has been edited to correct some things from the original posting and add a couple of links. Serious Eats  lists some more congresspeople participating.]
Last year the most popular food plan experiment was “eating local”. This year so far it seems to be “eating on a food stamp budget”. The main reason for this is upcoming debate on the 2007 farm bill . Bush administration is proposing to make big cuts  in food assistance for the poor, a large part of which would mean cuts to the food stamp program. [Edit: as an anonymous commenter pointed out, that was a link to an article about the 2005 farm bill cuts.] (A NY Times editorial about the subject  [Edit: this actually is about the 2007 Farm Bill :)].) So a number of politicians are doing the Food Stamp Budget Experiment at least in part to protest against this.
Here are the ones I’ve found so far (Note, some of these links were already posted to my del.icio.us, so my apologies for the duplicates if you follow that also.)
All of these politicians seem to be sticking to the $3-a-day-per-person budget. As mentioned here previously , blogger Rebecca Blood is conducting a month-long experiment  in this vein, though she’s allowing herself a $74 per day week budget  (for two people), and adding the additional challenge of eating organic food. [Edit: see my followup post .] She seems to be doing quite well so far, eating lots of fresh vegetables, pulses and grains. On the other hand Councilman Eric Gioia, who went shopping  with the assistance of a mother who had raised a family on food stamps, and bought things like ramen noodles, white bread (cheaper than whole wheat), canned tuna, peanut butter, isn’t doing very well. I suspect that more people who are actually in situations where they have to rely on food stamps shop more like the councilman than like Rebecca.
It does strike me that the councilman’s diet sounds suspiciously like that of a typical ‘starving’ student’s though, which is probably why a lot of the commenters on this Digg post  seem to think it’s not a big deal.