Shouldn't form_group use an offset instead of an empty label?
<%= bootstrap_form_for(...) do |f| %>
<%= f.form_group do %>
<%= f.submit %>
<% end %>
<% end %>
results in:
<div class="form-group">
<label class="control-label col-sm-3"></label>
<div class="col-sm-9">
<input ... type="submit" value="" />
</div>
</div>
I would rather see an offset (as used in bootstrap documentation):
<div class="form-group">
<div class="col-sm-offset-3 col-sm-9">
<input ... type="submit" value="" />
</div>
</div>