I've lusted after abandoned bikes too, but can't justify taking them.

It's a funny thing about the note--it's not a bad idea I guess, but I can't help thinking it's useless. If the owner reads the note, it's probably because he's in the act of moving/using the bike--which you'd notice anyway and would generally mean that the bike isn't available. If the bike is actually abandoned, there'd be nobody to read the note.

If someone leaves a bike chained up outside for a while, they aren't generally going to be checking up on it even if they plan to use it later. You can never be sure.