How do I close all buffers that aren't shown in a window in vim?

I have a horde of buffers open in vim, with only a few of them open in split windows or on other tabs. Is there a way to close all but the ones that are currently visible in one of those splits or tabs?

Yet another take on this. Using the example given in the Vim help for tabpagebuflist() to get a list of the buffers that appear in a tab or window. I have the below in my .vimrc

function! DeleteInactiveBufs()
    "From tabpagebuflist() help, get a list of all buffers in all tabs
    let tablist = []
    for i in range(tabpagenr('$'))
        call extend(tablist, tabpagebuflist(i + 1))
    endfor

    "Below originally inspired by Hara Krishna Dara and Keith Roberts
    "http://tech.groups.yahoo.com/group/vim/message/56425
    let nWipeouts = 0
    for i in range(1, bufnr('$'))
        if bufexists(i) && !getbufvar(i,"&mod") && index(tablist, i) == -1
        "bufno exists AND isn't modified AND isn't in the list of buffers open in windows and tabs
            silent exec 'bwipeout' i
            let nWipeouts = nWipeouts + 1
        endif
    endfor
    echomsg nWipeouts . ' buffer(s) wiped out'
endfunction
command! Bdi :call DeleteInactiveBufs()

Here's an alternative solution you can drop in your .vimrc:

function! Wipeout()
  " list of *all* buffer numbers
  let l:buffers = range(1, bufnr('$'))

  " what tab page are we in?
  let l:currentTab = tabpagenr()
  try
    " go through all tab pages
    let l:tab = 0
    while l:tab < tabpagenr('$')
      let l:tab += 1

      " go through all windows
      let l:win = 0
      while l:win < winnr('$')
        let l:win += 1
        " whatever buffer is in this window in this tab, remove it from
        " l:buffers list
        let l:thisbuf = winbufnr(l:win)
        call remove(l:buffers, index(l:buffers, l:thisbuf))
      endwhile
    endwhile

    " if there are any buffers left, delete them
    if len(l:buffers)
      execute 'bwipeout' join(l:buffers)
    endif
  finally
    " go back to our original tab page
    execute 'tabnext' l:currentTab
  endtry
endfunction

Use :call Wipeout().