i hope i haven't made a mistake while testing this problem (tried several times)
if i haven't it's quite a nasty one (and i'm not sure whether it is present in 1.4.5 or not).

create a new template, create one item in it.
create new host, link the host to the template.
in the template, create a trigger with this item.
make the item change the state so that the trigger goes TRUE.
disable the original item.
create a new host, link it to the same template.
verify that item condition warrants the trigger to be TRUE, notice that it does not appear in triggers page.
check that associated item is actually enabled in original template and the new host.
notice how the trigger does not appear even if 'show all' is selected in triggers page.
notice how the item is correctly collecting data in 'latest data' page.

i believe this i a very, very nasty bug, because it silently makes trigger useless and is quite hard to notice.