I think that would be only possible client-sided. Like the fly-mod. So I think there is no way to make a bukkit plugin for that.
sorry mate thats pretty much impossible without something like spout client, and everyone on the server would need ne client so thats a BIG no no
Ehm... Why exactly should it be impossible to manipulate the players velocity and it it's impossible why are there API calls like setVelocity() in bukkit? Also there are server-side flying mods...
@Afforess: And this is useable without a client mod (spoutcraft)? But even if so I think that function would just manipulate the players velocity, so... We could do that withoug spout, too.