This is a macro I've been using for thorns after the 4.0.1 patch. Its basically a tricks of the trade macro with thorns replacing tricks of the trade. Set the tank as your focus and then hit it. This is of course most useful to a Balance druid (for whom I made it), but resto can get use out of this as well.
Feral doesn't get much use out of it as thorns forces the druid into a caster form. But if you're a kitty DPS its useful as long as you realize you'll have to shift back into kitty form.
Tricks of the Thorns
/cast [target=focus] Thorns
Description: Set your intended target (usually a tank) as your focus target. Then click the macro and it'll automatically cast thorns on the tank without breaking your current target or requiring you to scroll through party or raid frames.