In the US, are the police always involved in suicides (potential or otherwise)? Here in the UK an ambulance would be called instead, and from (my own, regrettable) experience the police are not involved unless there are suspicious circumstances.
Yes, in the US, the police are typically called. Quite often police are called even if an actual crime is not committed because their role is often considered to be more than just crime-related.

And I do hope you never try anything like that again. Life can be difficult, but there's almost always a way to get out of any situation, if you put your mind to it.
After eight tries I'm beginning to think I have nine lives...
I've come a long way since then. Too far to turn back.

I'm a lot safer where I am now. I'm in the position where I can pick up the phone and Matt can be here within the hour if I need him, and living on my own I've made sure that nothing I could use to hurt myself comes through the door.
A suicide request is a message that arrives either by email to or through the Support board (although the latter is quickly closed) alerting the LJ abuse team to a journal entry that could be deemed as a suicide note.
What if the entry is really ambiguous, like "So long, and thanks for all the fish"? or "wow ... so little time, so much blood"?
This is why it's handled by five people on a case by case basis. Most suicide requests are made by friends of the person in question whom assumedly know a little bit about the person's life, enough to know when they are being cryptic, or the difference between something like self-harm and a potential suicide attempt, and presumably the LJAbuse team look through journals for private entries or anything written previously to suggest that someone was in such a state of mind.

It's by no means an easy, clear cut task, which is why it is quite difficult to become an LJAbuse Team volunteer because of the nature of their work (and the privs they have).

Now I know that if I want to kill myself, I won't say anything in my journal or I may be thwarted.

Okay, good, at least I'm not the only one who thought that. :p

Really though, all in all, it's a good policy (and surprising, as well, actually).
Rolling Stone
Latest issue of Rolling Stone features a story about a young woman who was murdered by her two friends. At least one had a Live Journal here. They threatened each other continuously (not necessarily here, but via email & IMs), and one definitely posted suicidal messages. Some of the posts are reprinted in the Rolling Stone article.

I regret that we can't reach out from the virtual world more often to help & save people, but I'm grateful for the effort.