Hotkey toggle for auto-attack?
ctrl+right click
You can set it to any skill on your bar that isn’t ground targetted or disable it completely. You can even have it enabled for one weapon set and disabled for another on the same character.
Works with conjured weapons too but it doesn’t remember the settings for environmental weapons like the ones at Claw of Jormag (which is a slight inconvenience btw).
(Ctrl+Right Click) only affects Auto-attack for skills which I assume isn’t what you’re asking for. As far as I know you can’t toggle auto-targeting on/off without going into the options and changing it manually each time. If someone does know a way I’d be interested as well since I prefer it off for melee and on for ranged.
http://wiki.guildwars2.com/wiki/Auto-attack
http://wiki.guildwars2.com/wiki/Auto-targeting