You mean those really funny looking guys, in the poorly put together T-51b. I don't think there is an American counterpart, but the costumes we make look a lot better.
http://www.google.com/imgres?q=fallout+larping&um=1&hl=en&sa=N&rls=com.microsoft:en-us:IE-SearchBox&biw=1280&bih=856&tbm=isch&tbnid=vCXYYT1-ORBhKM:&imgrefurl=http://englishrussia.com/2009/07/06/fall-out-for-real/&docid=dtk8OKv-96cXuM&w=750&h=499&ei=DnFGTueeBaTg0QGeu5yMCA&zoom=1&iact=rc&dur=157&page=1&tbnh=138&tbnw=184&start=0&ndsp=34&ved=1t:429,r:1,s:0&tx=49&ty=83 look at the girl in the background.
http://www.google.com/imgres?q=t-51b+costume&um=1&hl=en&sa=N&rls=com.microsoft:en-us:IE-SearchBox&biw=1280&bih=856&tbm=isch&tbnid=xTsEjPikfidGNM:&imgrefurl=http://www.therpf.com/f24/fallout-t-51b-helmet-58288/&docid=t-3LU5mkhJlggM&w=694&h=722&ei=rXBGTpW_Iqne0QHP1fGACA&zoom=1&iact=rc&dur=250&page=1&tbnh=144&tbnw=132&start=0&ndsp=31&ved=1t:429,r:8,s:0&tx=48&ty=25, the creator from Louisiana.