The problem with remote forms is that they use onsubmit to submit them. Unfortunately, Javascript does not fire onsubmit() when submit() is called :(
This causes problems with form_remote_tag in Rails, because that uses the onsubmit handler to work its Ajax magic. However, if you try to use Javascript to submit these forms (which I like to do from onchange handlers in select boxes), the onsubmit() function doesn't get fired.
Therefore, to work around, I use the following code:
collection_select("objname", :key_method, the_collection,
:id_accessor, :text_accessor, {}, {:onchange =>
"if(onsubmit()) { submit(); }"})
I haven't tested this beyond FireFox, but it seems to be working so far.
UPDATE: A more cross-browser version of the last parameter is shown below (submitted by a reader):
{ :onchange => "var f = document.forms['#{form_id}'];
f.onsubmit();" }